Add back a helper pref that holds a system setting.

Change-Id: I7d8978829bde1f1dd15bf59f464557f64aa07beb
Settings: add double-tap to sleep in Advanced Settings

Adapted for cm-12.0

Conflicts:
	res/values/cm_strings.xml
	res/xml/display_settings.xml
	res/xml/status_bar.xml

Change-Id: I910eb6624536b437d0bf14f64c1eeacafaa59cfa