Bass: Extract private removeSourceInternal from public removeSource

Public removeSource for removing a Broadcast Source from a Broadcast
Sink. It also supports group removal if addSource was previously used
with a group. Designed for external use, this method always removes
sources along with their cached values (mPausedBroadcastSinks,
mBroadcastMetadataMap), even if they were suspended, as this is
intended by the user.

Private removeSourceInternal, which is responsible for removing source
from a single sink device, without modifying cached values.

Use enqueueSourceGroupOp only on success in removeSourceInternal.

Bug: 391259349
Flag: EXEMPT, day-to-day bugfix covered by unit tests
Test: atest BassClientServiceTest
Change-Id: Ief1e0146e1417803bbbc5b7cde18ab5ac60e76b0