Listener to get the user id for this device. The
onUserIdReceived(String) method will be called
on the same thread that
Implement this to get the userId once it's sent from the server.
|public static final AtomicReference<Handler>||handler||This creates a Handler using the android.os.Looper of the thread that the callback is created on|
|abstract void||onUserIdReceived(String userId)|
This creates a Handler using the android.os.Looper of the thread that the callback is created on