Sets focus before sending a11y focus event in Android

[chunhtai]

In the original issue, the slider is not announce because android think it is not focused when it receive the focus event. This PR makes sure the a11ynodeinfo returns isAccessibilityFocus = true before sending out a11y event

Fixes flutter/flutter#79528

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[blasten]

LGTM

[blasten]

nit: is this class necessary? it could also use ArgumentCaptor to extract AccessibilityEvent

[chunhtai]

I need to verify the focus is set at the exact moment the requestSendAccessibilityEvent is called

[chunhtai]

so i need a callback to run when requestSendAccessibilityEvent is called

[chunhtai]

See line 845

[WillowWisp]

Hi, has this change been on stable channel already?
I'm still getting the bug #79528 . Thanks.

Edit: This doesn't work on beta channel either.

flutter doctor -v

[✓] Flutter (Channel beta, 2.8.0-3.2.pre, on macOS 11.6 20G165 darwin-x64, locale en)
    • Flutter version 2.8.0-3.2.pre at /Users/nguyenthang/Desktop/ThangDev/FlutterSDK/flutter_macos_2.8.0-3.2.pre-beta
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 2901cd7208 (6 days ago), 2021-11-18 11:15:09 -0800
    • Engine revision bcc2b7f12c
    • Dart version 2.15.0 (build 2.15.0-268.18.beta)

[✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at /Users/nguyenthang/Library/Android/sdk
    • Platform android-31, build-tools 31.0.0
    • ANDROID_HOME = /Users/nguyenthang/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 13.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • CocoaPods version 1.11.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
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    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)

[✓] VS Code (version 1.62.3)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.28.0

[✓] Connected device (2 available)
    • CPH2219 (mobile) • a2e11d62 • android-arm64  • Android 11 (API 30)
    • Chrome (web)     • chrome   • web-javascript • Google Chrome 96.0.4664.55

• No issues found!