AudioService: fix system stream muted by restore

Releases prior ICS (included) would persist the stream volumes in the
database when muted by silent mode. If the DB was backed up while in
silent mode, stream volume was backed up at 0.
When restored on a new device, the volume was restored at 0 which was
a problem for SYSTEM stream because it would never be corrected as
this stream volume is fixed.

Added a check on valid volume values when reading settings from the DB
and correct the DB if this happens.

Change-Id: Ie3b98eb74e10413c22aab7568b83ac3ace5bfc07