[Audiosharing] Add dynamic stream for audio sharing

Part 3: collect secondary headset volume changes in volume dialog

When sharing audio with two headsets, add a dynamic stream to control
the volume of secondary headset while keep using STREAM_MUSIC to control
primary headset.

Test: atest
Bug: 336716411
Flag: com.android.settingslib.flags.volume_dialog_audio_sharing_fix
Change-Id: Ifebdbc461a16cb522d6021debe52a93127fda50c