Route volume commands whenever media stream is in use

Found a regression in volume handling. Previously we handled
volume commands as long as the media stream was active but we were only
handling them when there was an active session on L. This adds a check to
make sure we handle volume if anything is playing on the media stream.

bug:17498479
Change-Id: Iddd745c8a762cf7ebedb37f1b26fc934db01fba0