title: Getting GCM Registration ID using Firebase
body: <p>I have an ASP.net MVC server from where I want to send push notifications to my Android App. I have already implemented <code>Firebase Messaging</code> in the app and the notifications are working fine when sent from Firebase dashboard.  </p>
  
  <p>I have tried sending push notification using the server by sending a post request, but the request requires a <code>to</code> field. Earlier we used to send <code>registration id</code> provided by <code>GCM</code> there. Now since Firebase is handling it, how can I fetch the registration id to be put in the to field using Firebase SDK in Android?</p>