VolumeZen: Sound & Notifications settings.

- Add sound settings under the newly combined Sound & Notifications
  top-level settings page.
- New slider preferences for setting audio stream volumes.
- Migrate checkbox prefs to switch prefs.
- New settings subpage for touch sounds.
- New settings subpage for "Showing notifications"
- New setting pref for displaying intercepted notifications in zen mode.
- New tri-state lockscreen notifications pref.
- Add priority + sensitivity options to app notification dialog.
- Remove badges on app notifications row, migrate to subtitle text.
- Migrate strings to common spot in strings.xml
- Remove heads up setting.
- Remove tweaks category (unused).
- Clean up notification settings, separate out into a code section per option.
- Clean up _settings.xml files, prefix each pref with option name.
- Add appropriate indexing data to each settings page.

Depends on f/b change Ic30aa575ae07650cee62c8c1d83b6bc69395cf0d

Change-Id: If700385a7d0057f6c4c4bcf6e648dcf88b8ebff2