Channel settings updates

- Update master switch preference to store enabled state in case
it's set before the view holder is bound
- Update master switch preference to prevent disabled switches
from being toggled
- Show an importance summary for max importance
- properly handle null notification sounds

Change-Id: I395b95b76d2a3c1c94b41d1c2720bb0cf1cae917
Fixes: 36920159
Fixes: 37421928
Fixes: 36939825
Test: RunSettingsRoboTests & manual