Avoid double-sending Play on UPnP track changes
Add LAST_PCM_TRACK_PATH to record the track path that pcm_upnp_start
sent SetAVTransportURI for (without Play). pcm_upnp_start now records
the path and, on subsequent starts, distinguishes track changes from
resumes: on a track change it issues SetAVTransportURI at the PCM
boundary without Play. The monitor checks LAST_PCM_TRACK_PATH and only
sends Play as a fallback if the PCM-boundary update wasn't performed.