title: Check if device is offline with firebase cloud messaging
body: <p>When sending messages via Firebase Cloud Messaging, the repsonse from FCM servers tell about the processing of the Request but not whether it was delivered or not to the device.</p>
  
  <p>So, for offline devices how could we know whether message was delivered?</p>
  
  <p>For example, </p>
  
  <blockquote>
    <p>{"multicast_id":7138524052704576147,"success":1,"failure":0,"canonical_ids":0,"results":[{"message_id":"0:1472139369768700%d42090b5d42090b5"}]}1</p>
  </blockquote>
  
  <p>the above JSON is a response from FCM server for request with time_to_live:0 and priority:'high' but the situation is that device for which I want to receive the message is offline.</p>
  
  <p>Is there any possible way to detect if the device is offline?</p>