Add back a helper pref that holds a system setting.

Change-Id: I7d8978829bde1f1dd15bf59f464557f64aa07beb
Use SystemSettingSwitchPreference.

Change-Id: I14fe1c9b1c5ead1ddb20c61ba080bb005f2130d7
Port "Option to control cursor in text fields using volume keys (2/2)"

Change-Id: I577e2d200c9da9d86db4f4892a60851978c9509e