Notification importance/selected sound conflict fixes.

- When you've selected "Silent" as your sound, update notification
importance messaging to match.
- When you explicitly move from a silent type of notifications to a
loud type, but have "Silent" selected as the sound, change the
sound to the default notification sound also.

Change-Id: I462785d593e1d6c7d1e87388aeee1bdcbcf6aa3d
Fixes: 63109928
Test: RunSettingsRoboTests, manual