How could I send messages from the cloud to Android?

Initialize the service in your application in the OnCreate method

Parse.initialize(this, "YOUR_APP_ID", "YOUR_CLIENT_KEY");

Enable to receive notification

ParsePush.subscribeInBackground("", new SaveCallback() {
  @Override
  public void done(ParseException e) {
    if (e == null) {
      Log.d("com.parse.push", "successfully subscribed to the broadcast channel.");
    } else {
      Log.e("com.parse.push", "failed to subscribe for push", e);
    }
  }
});