bluetooth: Report a2dp_source volume changes to the source device
Write the current volume to the `Volume` DBus property to keep the volume on the remote in sync. Without this the remote device shows the wrong volume, and any attempts to change it will cause an unexpected jump when the local volume has also been adjusted. Thanks to prior investments to improve volume synchronization, setting up callbacks and sending initial volume to the peer for HFP/HSP implementing this feature is as easy as unconditionally assigning a valid function to `set_source_volume`. `source_setup_volume_callback` is already responsible for attaching a `SOURCE_VOLUME_CHANGED` hook and sending initial (restored) volume to the peer (signifying support for Absolute Volume - if not derived from the presence of FEATURE_CATEGORY_2 on the profile yet).
Showing with 35 additions and 5 deletions