Material Settings on pre-M devices

Bug: 28848704
Bug: 28870690

- Fixed a crash caused by using getChildFragmentManager() in
  PrefsFragment.
- Fixed an issue where the timer ringtone summary would not update
  immediately.
- Fixed an issue where the back button would remove the PrefsFragment
  instead of closing the SettingsActivity.

Change-Id: I1a82010b975fe159dd40d96fafbdea4a3bb86183