LeAudioService: Use ReentrantReadWriteLock

Currently, in LeAudioService we use synchronized blocks to acces
mGroupDescriptors list. Gradually, as we wrapped more and more code in
synchronized blocks, deadlocks and TimeoutExceptions started popping
around.

Examples include BT stack calling AF from synchronized block, and
then AF calling BT stack back, to check some property of LE Audio
device.

mGroupDescriptors are modified just in 3 places, so accessing it is
perfect candidate for usage of ReentrantReadWriteLock - it should solve
all the deadlocks.

Bug: 325674688
Bug: 326295400
Test: atest BluetoothInstrumentationTests
Test:
  1. Bond, unbond, reconnect LE Audio device, preferably one using CSIP.
  2. Observe logcat output and verify no TimeoutException is thrown from LeAudioService
Change-Id: Icdb944981ddceca39df95694933d05ffe78ee72e