[Audiosharing] Add dynamic stream for audio sharing

Part 2: convert audio sharing callbacks to secondary headset volume changes

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: Ibdf66368c32b3fc3b84a333e4446f02ef75fe684