Priority only dnd doesn't change ringer

Streams are muted in AudioService#muteRingerModeStreams,
so there's no need to change the ringer when entering
priority only dnd.  Note: total silence and alarms only will
still change the ringer to silent.

Fixes: 76152626
Test: ZenModeHelperTest
Test: manual (toggle dnd on when ringer set to vibrate/normal
and observe the ringer icon remains the same with dnd badge)

Change-Id: I56cb90928362c18f8bae60c17f313a8a2621dd28