Priority only DND updates

- Changing ringer mode should put the device into
priority only mode, not alarms only mode

- The volume dialog will reflect the external ringer
mode instead of the internal ringer mode to better
represent what will happen to notification and other
sounds

Bug: 63096355
Test: manual, toggle ringer button in various dnd states
Change-Id: Iae09f50ece8e089752614d1c856ce9a118747048