Add back a helper pref that holds a system setting.

Change-Id: I7d8978829bde1f1dd15bf59f464557f64aa07beb
Use SystemSettingSwitchPreference.

Change-Id: I14fe1c9b1c5ead1ddb20c61ba080bb005f2130d7
Merge branch 'cm-12.0' of git://github.com/CyanogenMod/android_packages_apps_Settings into cm-12.0