[le audio] Improve bassClient sync data to be based on broadcast BIG

In current design, some assistant sync related data structures are based
on device. this is not extendable and will cause issue when remote source has multiple BIGs.

This commit modified those structures to be per broadcast id, so that we
can allow the sync to be per BIG and extend our support in the future.

Bug: 298662882
Test: atest BassClientServiceTest BassClientStateMachineTest
Test: manual test with broadcast
Change-Id: I67c7fea0b83ac68e169ef29bafe2f721655e9ddf