Add ToggleableGlobalSettingsFlag for QUICK_SWITCH

- ToggleableGlobalSettingsFlag saves/loads from Settings.Global rather than
  SharedPrefs, allowing other processes to update the setting as well.
- ToggleableGlobalSettingsFlag#get() reads from storage every time.
- Add the flags' descriptions to the feature flags settings UI.

Change-Id: I9cfc44ddaa5d9e223cf77a9b2475f8d25757d1fb