AudioService: fix errorprone LockOnNonEnclosingClassLiteral

Introduce new lock to synchronize VolumeGroupState /
VolumeStreamState operations instead of locking on
VolumeStreamState.class

Bug: 341995844
Flag: EXEMPT code cleanup, no functional change
Test: m -j RUN_ERROR_PRONE=true services | tee error_prone_services.txt
Change-Id: I3d0ecb9e8c4ad0a91118f398b9581b09b0d0bcae