[le audio] BassClient adding support for switching broadcast source

Currently if the sinks have active source, assistant won't proceed with
syncing or adding with new source.

This commit adds the changes for
1. Search and show remote source should not depend on whether remote
   buds are having source(either active or not).
2. Upon adding a source, stack will try to automatically remove any
   source(either active or not) to make a room first, and then proceed
   with adding the new source.

Bug: 316423019
Bug: 316005152
Test: atest BassClientServiceTest BassClientStateMachineTest
Test: manual test with le audio broadcast QR code path
Change-Id: I51cbe48b442cad6bddd2671d8fcdc590391d69d9