audio-groups, main-volume-policy, volume-api: Various fixes

audio-groups, main-volume-policy, volume-api: Various fixes

Sorry, this is a huge unreviewable commit. Contained improvements
include at least:

 * Flat volumes are now handled properly. Previously, audio groups
   controlled the absolute volume of streams if flat volume was in
   effect, which made no sense.
 * Audio group volumes are now persistent.
 * Audio group volumes are applied to new streams before the streams
   start to play, instead of after, which could cause audible
   glitches.
 * When a stream volume is changed by the user, the volume is
   propagated to the stream's audio group.
 * Fixed the handling of the "NEG" keyword in the match syntax in
   module-audio-groups. Previously the "NEG" keyword was parsed, but
   it had no effect.

Change-Id: I02bad3d23b3e562c71dbc6af6f3e308089893751