bass_client: Remove pending source operation when stopping receivers

Some sink devices may not process add source message before stopping
synchronization. Not removing pending operation may lead to block next
add source operation. For external broadcas a timer for source operation
is introduced that would remove pending operation in case of no source
action (add/modify).

Tag: #bug
Bug: 333050419
Bug: 332847049
Test: atest BassClientStateMachineTest BassClientServiceTest
Change-Id: I00967fbcc2b65d17eb23ca8aac6d6c51db1e5bd6