title: Get all subscribed topics from firebase cloud messaging
body: <p>Using the new FirebaseMessaging it's easy to un/subscribe to topics via:</p>
  
  <pre><code>FirebaseMessaging.getInstance().subscribeToTopic();
  FirebaseMessaging.getInstance().unsubscribeFromTopic();
  </code></pre>
  
  <p>But is there any way to get all topics the current installation is subscribed to ?</p>