Add lock_guard to FetchAudioProvider.

Audio provider are refetched mainly due to the service being restarted.
Adding lock_guard to this function prevent service access while
refetching for other important functions (StartSession, EndSession, ...)
to prevent BT crashes.

Also add lock_guard access to some other important functions.

Bug: 341856622
Test: mmm packages/modules/Bluetooth
Flag: EXEMPT trivial change

Change-Id: I0e8ad04414dd79105ad8339e866b82a7d2ab8ad2