AudioService: assistant UID has its own lock

Continue moving independent information in AudioService under
their own locks to prevent lock nesting:
assistant UID related fields are guarded by new mAssistantUidLock.
Affects:
  mAssistantUids
  mPrimaryAssistantUid
  mActiveAssistantServiceUids

Bug: 287066735
Flag: EXEMPT code health
Test: atest AudioManagerTest
Test: m -j RUN_ERROR_PRONE=true services | tee error_prone_services.txt
Change-Id: I8517b7255081920bb7fdc38204d671414a02879b