audio_output/winstore: report initial volume/mute status

Previously, the volume and mute properties would read their default values (0 and false respectively) since neither Open() or Start() would call aout_{Volume,Mute}Report by themselves. Now, the initial volume and mute status are read during Start(), which means that setting the volume should work after the first volume_changed event is received by a libvlc consumer.

Partial workaround for #26032.

As a result, reporting the volume level after (un)muting is no longer necessary (d104faec).