Bass: Fix ConcurrentModificationException on mPausedBroadcastSinks

Disconnecting the remote during resumeReceiversSourceSynchronization
can cause a ConcurrentModificationException on mPausedBroadcastSinks.
Fixed by changing HashSet to ConcurrentHashMap.newKeySet().

Bug: 396069263
Flag: EXEMPT, day-to-day bugfix covered by unit tests
Test: atest BassClientServiceTest
Change-Id: Ida37d4470e99c45cd01243edb8270ccb2a342251