BTA: turn service discovery events into callbacks

Instead of single callback, that handles multiple, mostly unrelated
events, we should have callbacks for each event.

This would reduce mental complexity of following this code - instead of
finding event, associated struct, invoker and callback, one would have
to just do single search.

Bug: 330675788
Flag: exempt, no logic changes, hard to isolate architecture refactor
Test: mma -j32
Change-Id: I055358d95278b2e2522b403607d9bbde96bbc695