[ci] Add a legacy Android build-all test (flutter#4005)

Adds the ability to replace portions of the `flutter create`d app with saved copies, and adds a second build-all phase for Android that uses a Flutter 2.0.6-created `android/` directory (AGP 4.1, Gradle 6.7) to catch issues like flutter#125621 and flutter#125482 prior to release.

Includes some incidental cleanup:
- Extracts a helper method for adjusting files, so that this doesn't add even more copies of basically identical code.
    - (This was motivated by an earlier version of the PR that added modifications to several more files, which I ended up undoing, but the cleanup seemed worth keeping.)
- Adds missing unit test coverage.
- Reworks the unit tests to use a mock process manager and dummy files, instead of each test actually calling `flutter create`, which made each new test add several seconds to the unit test suite.
    - While this reduces the integration-style coverage, in practice the integration tests of the repo tooling is the CI itself, so the unit tests should be true unit tests.
- Changes the non-legacy test to Kotlin; we were still testing with Java even though Kotlin has been the default for quite a while, so we weren't testing what most new users would actually be running. Since we now have a legacy test, I used Java there to cover both.
- Removes some dead code for modifying the AndroidManifest.xml; when trying to set up unit tests for it I discovered that it no longer matches anything in an actual project. It dates back to the original command, and seems to have been a camera-related hack, which we clearly no longer need since it wasn't working and camera still works in build-all.

This is captured somewhat in the README in the legacy project directory, but to document the approach here: originally I was going to add flags to change individual items (AGP version, Gradle version), but quickly ran into the fact that selective downgrading is extremely fragile. E.g.,:
- The Kotlin version set in current projects doesn't work when downgrading AGP and Gradle.
- The app template can unconditionally use anything (e.g., `namespace`) that the AGP version it uses supports, so arbitrary future breakage is possible.

It's also less useful as a real test of what a plugin client's project likely looks like. Starting with a complete platform directory, and doing whatever the minimal changes are to keep it working, will likely reflect a common real-world scenario. On the flip side, the reason this doesn't use a complete 2.0.6 project, but instead is based on specific platform directories, is that we don't want to waste time manually maintaining, e.g., old Dart code that is irrelevant to the goal of the test. For now this only uses Android because that's where we've seen problems in practice, but we can alway add other legacy platform tests later if we find a need.

Fixes flutter#125689

Author: stuartmorgan-g

.ci/legacy_project/README.md

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+This directory contains a partial snapshot of an old Flutter project; it is
+intended to replace the corresponding parts of a newly Flutter-created project
+to allow testing plugin builds with a legacy project.
+
+It was originally created with Flutter 2.0.6. In general the guidelines are:
+- Pieces here should be largely self-contained rather than portions of
+  major project components; for instance, it currently contains the entire
+  `android/` directory from a legacy project, rather than a subset of it
+  which would be combined with a subset of a new project's `android/`
+  directory. This is to avoid random breakage in the future due to
+  conflicts between those subsets. For instance, we could probably get
+  away with not including android/app/src/main/res for a while, and
+  instead layer in the versions from a new project, but then someday
+  if the resources were renamed, there would be dangling references to
+  the old resources in files that are included here.
+- Updates over time should be minimal. We don't expect that an unchanged
+  project will keep working forever, but this directory should simulate
+  a developer who has done the bare minimum to keep their project working
+  as they have updated Flutter.
+- Updates should be logged below.
+
+The reason for the hybrid model, rather than checking in a full legacy
+project, is to minimize unnecessary maintenance work. E.g., there's no
+need to manually keep Dart code updated for Flutter changes just to
+test legacy native Android build behaviors.
+
+## Manual changes to files
+
+The following are the changes relative to running:
+
+```bash
+flutter create -a java all_packages
+```
+
+and then deleting everything but `android/` from it:
+
+- Added license boilerplate.

.ci/legacy_project/all_packages/.gitignore

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+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+**/ios/Flutter/.last_build_id
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+/build/
+
+# Web related
+lib/generated_plugin_registrant.dart
+
+# Symbolication related
+app.*.symbols
+
+# Obfuscation related
+app.*.map.json
+
+# Android Studio will place build artifacts here
+/android/app/debug
+/android/app/profile
+/android/app/release

.ci/legacy_project/all_packages/.metadata

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+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
+  channel: unknown
+
+project_type: app

.ci/legacy_project/all_packages/android/.gitignore

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+gradle-wrapper.jar
+/.gradle
+/captures/
+/gradlew
+/gradlew.bat
+/local.properties
+GeneratedPluginRegistrant.java
+
+# Remember to never publicly share your keystore.
+# See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app
+key.properties

.ci/legacy_project/all_packages/android/app/build.gradle

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+def localProperties = new Properties()
+def localPropertiesFile = rootProject.file('local.properties')
+if (localPropertiesFile.exists()) {
+    localPropertiesFile.withReader('UTF-8') { reader ->
+        localProperties.load(reader)
+    }
+}
+
+def flutterRoot = localProperties.getProperty('flutter.sdk')
+if (flutterRoot == null) {
+    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
+}
+
+def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
+if (flutterVersionCode == null) {
+    flutterVersionCode = '1'
+}
+
+def flutterVersionName = localProperties.getProperty('flutter.versionName')
+if (flutterVersionName == null) {
+    flutterVersionName = '1.0'
+}
+
+apply plugin: 'com.android.application'
+apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
+
+android {
+    compileSdkVersion 30
+
+    defaultConfig {
+        applicationId "com.example.all_packages"
+        minSdkVersion 16
+        targetSdkVersion 30
+        versionCode flutterVersionCode.toInteger()
+        versionName flutterVersionName
+    }
+
+    buildTypes {
+        release {
+            signingConfig signingConfigs.debug
+        }
+    }
+}
+
+flutter {
+    source '../..'
+}

.ci/legacy_project/all_packages/android/app/src/debug/AndroidManifest.xml

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+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.example.all_packages">
+    <uses-permission android:name="android.permission.INTERNET"/>
+</manifest>

.ci/legacy_project/all_packages/android/app/src/main/AndroidManifest.xml

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+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.example.all_packages">
+   <application
+        android:label="all_packages"
+        android:icon="@mipmap/ic_launcher">
+        <activity
+            android:name=".MainActivity"
+            android:launchMode="singleTop"
+            android:theme="@style/LaunchTheme"
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
+            android:hardwareAccelerated="true"
+            android:windowSoftInputMode="adjustResize">
+            <!-- Specifies an Android theme to apply to this Activity as soon as
+                 the Android process has started. This theme is visible to the user
+                 while the Flutter UI initializes. After that, this theme continues
+                 to determine the Window background behind the Flutter UI. -->
+            <meta-data
+              android:name="io.flutter.embedding.android.NormalTheme"
+              android:resource="@style/NormalTheme"
+              />
+            <!-- Displays an Android View that continues showing the launch screen
+                 Drawable until Flutter paints its first frame, then this splash
+                 screen fades out. A splash screen is useful to avoid any visual
+                 gap between the end of Android's launch screen and the painting of
+                 Flutter's first frame. -->
+            <meta-data
+              android:name="io.flutter.embedding.android.SplashScreenDrawable"
+              android:resource="@drawable/launch_background"
+              />
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN"/>
+                <category android:name="android.intent.category.LAUNCHER"/>
+            </intent-filter>
+        </activity>
+        <!-- Don't delete the meta-data below.
+             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
+        <meta-data
+            android:name="flutterEmbedding"
+            android:value="2" />
+    </application>
+</manifest>

.ci/legacy_project/all_packages/android/app/src/main/java/com/example/all_packages/MainActivity.java

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+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package com.example.all_packages;
+
+import io.flutter.embedding.android.FlutterActivity;
+
+public class MainActivity extends FlutterActivity {
+}

.ci/legacy_project/all_packages/android/app/src/main/res/drawable-v21/launch_background.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Modify this file to customize your launch splash screen -->
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:drawable="?android:colorBackground" />
+
+    <!-- You can insert your own image assets here -->
+    <!-- <item>
+        <bitmap
+            android:gravity="center"
+            android:src="@mipmap/launch_image" />
+    </item> -->
+</layer-list>

.ci/legacy_project/all_packages/android/app/src/main/res/drawable/launch_background.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Modify this file to customize your launch splash screen -->
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:drawable="@android:color/white" />
+
+    <!-- You can insert your own image assets here -->
+    <!-- <item>
+        <bitmap
+            android:gravity="center"
+            android:src="@mipmap/launch_image" />
+    </item> -->
+</layer-list>