Added javadoc comments to FlutterActivity and FlutterFragmentActivity. (#12328)

shell/platform/android/io/flutter/embedding/android/FlutterActivity.java

@@ -187,7 +187,15 @@ import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.SPLASH_S
  *     android:value="true"
  *     />
  * }
+ * <p>
+ * <strong>Alternative Activity</strong>
+ * {@link FlutterFragmentActivity} is also available, which is similar to {@code FlutterActivity}
+ * but it extends {@code FragmentActivity}. You should use {@code FlutterActivity}, if possible,
+ * but if you need a {@code FragmentActivity} then you should use {@link FlutterFragmentActivity}.
  */
+// A number of methods in this class have the same implementation as FlutterFragmentActivity. These
+// methods are duplicated for readability purposes. Be sure to replicate any change in this class in
+// FlutterFragmentActivity, too.
 public class FlutterActivity extends Activity
     implements FlutterActivityAndFragmentDelegate.Host,
     LifecycleOwner {

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

@@ -53,6 +53,9 @@ import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.SPLASH_S
  * is considered to be the standard, canonical implementation of a Flutter
  * {@code Activity}.
  */
+// A number of methods in this class have the same implementation as FlutterActivity. These methods
+// are duplicated for readability purposes. Be sure to replicate any change in this class in
+// FlutterActivity, too.
 public class FlutterFragmentActivity extends FragmentActivity
     implements SplashScreenProvider,
     FlutterEngineProvider,