Sendspin-driven AirPlay speakers keep the volume they are playing at across a track change (#5677)

A track change reuses the running cliairplay process, which nothing re-sends VOLUME= over. The parent volume/mute sync now happens only where a fresh process is spawned, right before it connects, so a reused process keeps the volume and mute it is already playing at and a fresh one is always told the state it is about to be sent.