title: Android OneSignal setGroup null
body: <p>I'm trying to stack notifications with OneSignal on Android using the group key. Unfortunately we can't set it server-side, so I was trying to set it client-side with the setGroup() builder option in my NotificationExtenderService class: </p>
  
  <pre><code>    public class MyNotificationExtenderService extends NotificationExtenderService {
  
          @Override
          protected boolean onNotificationProcessing(final OSNotificationReceivedResult receivedResult) {
              OverrideSettings overrideSettings = new OverrideSettings();
              overrideSettings.extender = new NotificationCompat.Extender() {
                  @Override
                  public NotificationCompat.Builder extend(NotificationCompat.Builder builder) {
  
                      builder.setGroup("myGroup");
  ...
  </code></pre>
  
  <p>This doesn't seem to work, however, as the notifications are still separated, and if I attempt to get the group key in the NotificationOpenedHandler it is null. Anyone know how to properly stack OneSignal notifications client-side?</p>