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
Change-Id: Iec41c1f9c4633f084d8740732e549bbdb552bf5b