[le audio] BassClient multiple sources handling improvement

Current code doesn't handle multiple sources well. It might have race
conditions and cause unsynced source got reported, and following PAST
operation might fail because of this.

This commit improved the handling of multiple sources for BassClient,
1. We need to make sure all notified sources are managable by
   maintaining the syncs.
2. Limit the max active synced sources.
3. Clean up the source properly when source or sink lost.

Bug: 294456735
Tag: #bug
Test: atest BassClientStateMachineTest BassClientServiceTest
Test: manually test assistant with two sources
Change-Id: Ib69563a342f3f093997902fbf6252bd9626550b3