title: Send notification to only one user in firebase
body: <p>I have an android app and I want to send notifications to only one user when data is change. How can I do that? I don't want to send my message to all users of my app, just the user whose data changed.</p>
  <p>Since Firebase returns a unique registration token created for every device after client initialization, is there any way to send the notification to a particular user or only to a group of Users using that token?</p>