Fix concurrency issue in shared mem allocator

- Add locking to ScopedAllocator, since it is possible for remote
handles to be cleared from an arbitrary binder thread
- Minor templating cleanup in ScopedAllocation

Test: atest shared_memory_allocator_tests --host
Test: atest AudioTrackTest AudioRecordTest
Bug: 264359990
Change-Id: I5bb01067125891bb8a51898c37d5c9189b6625ba