code: void onSuccessfulRegistration(Context context, String regId) {
  
          // You should send the registration ID to your server over HTTP, so it
          // can use GCM/HTTP or CCS to send messages to your app.
          getGcmListener(context).sendRegistrationIdToBackend(regId);
  
          // Persist the regID - no need to register again.
          // Also serves for detection of previous registrations
          storeRegistrationId(context, regId);
      }
docs: Called from an IntentService background thread