code: public static void setDefaultPushCallback(android.content.Context context,
                                              java.lang.Class<? extends android.app.Activity> cls) {
      LOGGER.d("setDefaultPushCallback cls=" + cls.getName());
      startServiceIfRequired(context, cls);
      AVPushMessageListener.getInstance().getNotificationManager().addDefaultPushCallback(AVOSCloud.getApplicationId(), cls.getName());
    }
docs: Provides a default Activity class to handle pushes. Setting a default allows your program to
  handle pushes that aren't registered with a subscribe call. This can happen when your
  application changes its subscriptions directly through the AVInstallation or via push-to-query.
  
  @param context This is used to access local storage to cache the subscription, so it must
  currently be a viable context.
  @param cls     This should be a subclass of Activity. An instance of this Activity is started when
  the user responds to this push notification. If you are not sure what to use here,
  just
  use your application's main Activity subclass.