code: public static Observable<JSONObject> sendMessageInBackground(String message, AVQuery<? extends AVInstallation> query) {
      AVPush push = new AVPush();
      push.setMessage(message);
      push.setQuery(query);
      return push.sendInBackground();
    }
docs: A helper method to concisely send a push message to a query. This method is equivalent to
  
  <pre>
  AVPush push = new AVPush();
  push.setMessage(message);
  push.setQuery(query);
  push.sendInBackground();
  </pre>
  
  @param message The message that will be shown in the notification.
  @param query A AVInstallation query which specifies the recipients of a push.