LeAudio: Fix for IllegalStateException

It actually might happen when enabling/disabling bluetooth very quickly,
the registerCallback call will be executed when service is not
available.
In such a case, service should just ignore the call.

Inside this CL other places with same patern has been fixed.

Bug: 336152773
Flag: EXEMPT, fix for the stress tests
Test: atest BluetoothLeAudioTest LeAudioServiceTest
VolumeControlServiceTest HapClientTest

Change-Id: I931f72eaa7c1b67f657257da66dfc013f25fa4a2