Fixed android build tools compile setting

* And misc clean up

Author: jkasten2

build.gradle

@@ -1,12 +1,3 @@
-buildscript {
-    repositories {
-        maven { url 'https://maven.google.com' }
-    }
-    dependencies {
-        classpath 'com.android.tools.build:gradle:3.1.0'
-    }
-}
-
 plugins {
     id 'com.gradle.plugin-publish' version '0.9.10'
     id 'java-gradle-plugin'
@@ -19,12 +10,14 @@ repositories {
 
 apply plugin: 'groovy'
 apply plugin: 'java-gradle-plugin'
+apply plugin: 'java-library'
 
 dependencies {
     compile gradleApi()
     compile localGroovy()
 
-    compile 'com.android.tools.build:gradle:3.1.0'
+    compileOnly 'com.android.tools.build:gradle:3.1.0'
+    testCompileOnly 'com.android.tools.build:gradle:3.1.0'
 
     testCompile gradleTestKit()
     testCompile 'junit:junit:4.12'

src/main/groovy/com/onesignal/androidsdk/GradleProjectPlugin.groovy

@@ -122,9 +122,6 @@ class GradleProjectPlugin implements Plugin<Project> {
 
     static boolean didUpdateOneSignalVersion
 
-    // TODO: MUSTS BEFORE next release 0.8.3+
-    //   TODO: 1. Compat for VersionRangeSelector.intersect for Gradle 2.14.1 to 4.2
-    //   TODO: 2. Ensure VersionRangeSelector.accept works
     @Override
     void apply(Project inProject) {
         project = inProject

src/test/groovy/com/onesignal/androidsdk/MainTest.groovy

@@ -254,7 +254,7 @@ class MainTest extends Specification {
     def "Upgrade to compatible OneSignal SDK when targetSdkVersion is 26"() {
         when:
         def results = runGradleProject([
-            compileSdkVersion: 26, // Unexpected crash if this is 27...
+            compileSdkVersion: 27, // Unexpected crash if this is 27 and com.onesignal:OneSignal:3.5.+
             compileLines : "compile 'com.onesignal:OneSignal:3.5.+'",
             skipGradleVersion: '2.14.1' // This check requires AGP 3.0.1+ which requires Gradle 4.1+
         ])