libaudiohal@aidl: Fix deadlock when stream goes away

Split the lock for accessing 'DeviceHalAidl::mCallbacks'
from the main lock 'DeviceHalAidl::mLock'. This is needed
to handle the case when the code of 'Hal2AidlMapper'
becomes the sole owner of the stream and ends up destroying
it, triggering a callbacks cleanup call. Since all calls to
'Hal2AidlMapper' are protected by 'mLock', it can not be
used for implementing exclusive access to 'mCallbacks'.
To avoid introducing deadlocks due to use of two mutexes,
the new mutex 'mCallbacksLock' is used only to guard
access to 'mCallbacks', effectively making it a synchronized
map.

Bug: 357487484
Test: atest CoreAudioHalAidlTest
Change-Id: Ic7e5314ad62954f9f8347af2049a5216cab01b7f