audio service: fix system volume settings

STREAM_SYSTEM stream volume is never persisted to settings as
it is always derived from another stream volume (STREAM_RING on phones
and STREAM_MUSIC on tablets). Therefore values stored in settings
are stale from previous releases and should be ignored.

Also fix a problem where a muted stream can be unmuted by
readAudioSettings() even if it is muted by ringer mode.

Bug 7216630.

Change-Id: If23561ddfbc704f89fd0a997faf1b50299a50c2e