title: Firebase notification doesn't appear on device
body: <p>I pause my app in <code>OnTokenRefresh()</code>, copy the token, put the app into the background and then send a message using Firebase console and it says the token is not registered.</p>
  
  <p>The advice is that the device will only receive a notification if it's backgrounded, and yet the act of backgrounding the app seems to unregister the token.</p>
  
  <p>Why does this happen?</p>