Don't enable the light sensor if it's not being used

Turning the screen on always enabled the ALS, regardless of the
auto-brightness setting. This appears to be an error from
change Ib88310d642d498591e8879c8269d630ff3f7c8af, since
enableLightSensor only applies to the display, and I don't see how
this can affect the button backlights.

Change-Id: I5ec0e757602a10515ba3f1cfe10f38310d989ea6