title: Handle onMessageReceived() and onTokenRefresh() even if the app is not running
body: <p>I have an app with 2 services :</p>
  
  <pre><code>MessagingService extends FirebaseMessagingService
  </code></pre>
  
  <p>and</p>
  
  <pre><code>InstanceIDService extends FirebaseInstanceIdService
  </code></pre>
  
  <p>There are declared in the <code>AndroidManifest.xml</code> file.
  The problem is that when my app's process is forced close itself, the services are also killed.</p>
  
  <p>I'd like them not to be killed. I've heard about the <code>START_STICKY</code> flag for the <code>Service</code> class, but I can't override the <code>onStartCommand()</code> method in these services so as to return this flag...</p>