Update texts for Settings->Sound->Do not disturb

- Update title to "Do not disturb preferences"
- Add summary text to reflect the priority only allows preferences
- Refactor the summary handling in ZenModeSettings so that the
  controller can get the summary text

Fix: 34975939
Test: make RunSettingsRoboTests

Change-Id: I3ca1dc43fe1d943735a9f625e66c89708a18300a
5 files changed