Add a CheckboxPreference to enable/disable illumination bar

This option will be used to enable/disable illumination bar
upon hardware or softkey button press. Normally it'll not be
visible in settings unless users add ro.semc.illumination=true
in build.prop. This is done to avoid confusion on unsupported
device.

Conflicts:
	res/values/strings.xml
	src/com/android/settings/DisplaySettings.java