title: How to get Firebase user id from FCM token? (in admin code on server)
body: <p>My signed-in Android clients register their tokens with my server. The problem is I am not sure whether the token sent by a particular client genuinely belongs to that client user id. A bad client could register valid tokens of other users.</p>
  
  <p>Given an FCM token and a user id, how can my admin code on the server verify that the token indeed belongs to the (authenticated) user id?</p>