LeAudioService: Replace `synchronized(mGroupLock)` with ReentrantLock

So that we can flag the implementation to use ReentrantReadWriteLock
later

synchronized(object) {}

is equivalent to

mLock.lock();
try {} finally { mLock.unlock(); }

Bug: 325674688
Test: atest BluetoothInstrumentationTests
Flag: EXEMPT, mechanical refactor, no logical change
Change-Id: I666db6a8dd8e024f68e0b1092677a23744b16252