Make single volume devices more robust to ringer mode changes.

In single volume mode:
Make getNewRingerMode always return the existing one, as setRingerMode
does nothing when in single volume mode.
Check ringer mode change respects single volume mode.

Bug: 69814628
Test: flash to single volume mode device
Change-Id: I66ef173c97118fe471dec462d7a82ad2e62064b0
1 file changed