[AWARE][RTT] Check for null callback registration

Check on the manager side for null callback registration and throw
and exception.

Should be done in addition to service side checks since, in well
behaved apps (which don't bypass the manager), the callback registered
with the service is a proxy wrapper for the user-provided callback.

Bug: 71575804
Test: unit test & integration tests
Change-Id: Ice6585fccb13d842d9518b1f60bff911fb66c48f