Bass: Handle adding source before BASS ready

If adding source before BASS state ready, assistant will queue the
operation and resume to add once BASS get ready.
If BASS failed in service discovery or MTU change, assistant will notify
the client about the source add failure and remove the pending source.

Bug: 385845170
Flag: EXEMPT; trivial change covered by unit tests
Test: atest BassClientServiceTest BassClientStateMachineTest
Change-Id: I9fa73ed9f386e4939698b6316026ff45841397bc