Introduce SystemSettingsCheckBoxPreference

  -Originally part of a3342e2 (Add back notification
   light settings.)

Change-Id: I474a14a1b395407f5ca273a5d6a3a7f03f7e4698
Add back a helper pref that holds a system setting.

Change-Id: I7d8978829bde1f1dd15bf59f464557f64aa07beb