AudioService: persist volume of ring stream when ringer mode changed

-Originally, when ringer mode changed from vibrate to normal by
 adjusting volume from 0 to 1 , volume index will not be persisted.
 After device rebooted, volume of ring stream will be restored to 0
-Persist volume index when ringer mode changed

Change-Id: I75fbeaab01d80b8bcee623795f59a9e9f16b9634
(cherry picked from commit 6fc65dae)