Add settings for devices without backlight control

Adds a new setting for explicitly disabling backlight control in display
settings. This avoids a copy of display_settings.xml in LGE Star's
overlays (see I73c910444c921047a4dd4043ffda4275cd101bd2 for
devices/lge/star-common).

Also changes the string of the summary in the main screen to not mention
display timeout but rather backlight and adds a second string saying
the same but without backlight.

Change-Id: I488cc5a40cff9076abedf0492cd54d1ace263502