Not supported activityUpdateStream for android

[Igos19]

activityUpdateStream returns Stream.empty() for android

@override
  Stream<ActivityUpdate> get activityUpdateStream {
    if (defaultTargetPlatform != TargetPlatform.iOS) {
      return Stream.empty();
    }

    return activityStatusChannel
        .receiveBroadcastStream('activityUpdateStream')
        .distinct()
        .map((event) =>
            ActivityUpdate.fromMap(Map<String, dynamic>.from(event)));
  }