Make the Flutter Gradle Plugin apply FlutterExtension to (flutter) plugins (#151861)

Makes the Flutter Gradle Plugin apply the `FlutterExtension` (the class that vends `flutter.minSdkVersion`, etc) to Flutter plugins.

This allows plugin authors to use `flutter.compileSdkVersion` and the like in plugin `build.gradle` files.

Doesn't use it in templates yet - holding off till we can see how this works in the plugins repo (which will sadly have to wait till this makes the next stable � )

Author: gmackall

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -765,6 +765,9 @@ class FlutterPlugin implements Plugin<Project> {
         if (pluginProject == null) {
             return
         }
+        // Apply the "flutter" Gradle extension to plugins so that they can use it's vended
+        // compile/target/min sdk values.
+        pluginProject.extensions.create("flutter", FlutterExtension)
         // Add plugin dependency to the app project.
         project.dependencies {
             api(pluginProject)