Unregister AudioManager callbacks while LE Audio service stops

There is chance that AudioManager may call already cleared callback
instances from LeAudioService instance. To avoid this situation,
callbacks needs to be cleared when service stops.

Tag: #feature
Bug: 258573934
Test: atest BluetoothInstrumentationTests
Merged-In: Iec41c1f9c4633f084d8740732e549bbdb552bf5b
Change-Id: Iec41c1f9c4633f084d8740732e549bbdb552bf5b
(cherry picked from commit 626b786f)