title: Google Firebase Notifications for Android - when app is not running
body: <p>I am trying to implement a functionality wherein users get notification messages even when the app is not running (neither in foreground nor in background). Companies like Amazon do send notifications and they show up in the notification tray - when tapped, the app opens or whatever intent the message carried with it.</p>
  
  <p>I have been able to implement notification handling when the app is in foreground and background... but when the app is not running, there are no notification messages received at all!</p>
  
  <p>From what I searched around, I think there needs to be a Service running in the background that keeps listening to notification messages - because a service is destroyed when the app is closed. Am I going in the right direction?
  Can someone point me to some code that implements or highlights the same.</p>