fix(messaging): Method channel handlers were not created if FirebaseMessaging.instance.* is not invoked beforehand

[russellwheatley]

Description

For example, imagine we had this setup, and no other FirebaseMessaging API calls. None of the event handlers would ever fire on native platforms. If the line FirebaseMessaging.instance.isAutoInitEnabled is uncommented, they will work because it will instantiate MethodChannelFirebaseMessaging that creates the method channels necessary for them to work.

This PR will always create the method channel handlers before MethodChannelFirebaseMessaging is instantiated.

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);

  // Uncomment the line below and the event listeners will fire.
  // FirebaseMessaging.instance.isAutoInitEnabled;
  FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

  FirebaseMessaging.onMessage.listen((RemoteMessage message){
    
  });
  FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
    
  });

  runApp(MessagingExampleApp());
}

Related Issues

fixes #11008
fixes #11410
fixes #10043

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Breaking Change

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