Tie the lockscreen sounds with the ringer volume.

Actual volume is a ratio of the ringer volume and drops along with
but slower than the ringer volume, so that at lowest ringer volume,
the lockscreen sounds are still somewhat audible.

Don't start the sounds if the ringer is muted.

Bug: 5394473
Change-Id: Ifcf242b3198b4ec8f12334e26ec23ebf05a96b83