[Audiosharing] `onDestroy()` is not guaranteed to be called after...

[Audiosharing] `onDestroy()` is not guaranteed to be called after `stopSelf()`. In this case callbacks are not unregistered timely and the notification kept being brought up again if any callback is received.

This CL also moved some binder calls to bg thread.

Test: atest -c com.android.settings.connecteddevice.audiosharing.audiostream
Flag: com.android.settingslib.flags.enable_le_audio_qr_code_private_broadcast_sharing
Bug: 347605485
Change-Id: I1a3a3db88178a43f27cac74cf743bdb75cdfb60e