Add a deprecation note to FlutterFragmentActivity (#18865)

shell/platform/android/io/flutter/app/FlutterFragmentActivity.java

@@ -15,9 +15,14 @@ import io.flutter.view.FlutterNativeView;
 import io.flutter.view.FlutterView;
 
 /**
- * Base class for activities that use Flutter who also require the use of the Android v4 Support
- * library's {@link FragmentActivity}. Applications that don't have this need will likely want to
- * use {@link FlutterActivity} instead.
+ * Deprecated class for activities that use Flutter who also require the use of the Android v4
+ * Support library's {@link FragmentActivity}.
+ *
+ * <p>Deprecation: this class is replaced by {@link
+ * io.flutter.embedding.android.FlutterFragmentActivity}.
+ *
+ * <p>Applications that don't have this need will likely want to use {@link FlutterActivity}
+ * instead.
  *
  * <p><strong>Important!</strong> Flutter does not bundle the necessary Android v4 Support library
  * classes for this class to work at runtime. It is the responsibility of the app developer using