[Audiosharing] Fix volume control.

1. Unregister the volume control callbacks in onDestroy
2. Reuse SeekBarPreference#setProgress
3. Fetch volume from AudioManager if got invalid volume from VC service.

Flagged with enable_le_audio_sharing

Bug: 305620450
Test: manual
Change-Id: Id63d1dbff4c3cbdeb91c2537d951c39e1a932220