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

We want to fully disable the lightsensor when it's not used, for two reasons:
1 - Power savings
2 - Sensors that control the backlight directly need to be disabled when
brightness control is set to manual. Forcibly enabling the sensor when
the screen turns on makes these devices incapable of using manual
brightness.

Change-Id: I64c1114348ff096e0a72b265f6f141d1ecef249c

Fix button backlights that depend on ALS

On a few devices, lighting the buttons when the screen turns on
depends on the lightsensor, regardless of the user preference.
So go back to enabling the sensor whenever the screen turns on, and
enforce the user preference after running through the light settings
once.

Change-Id: I6f036a32315eb7688a9cd11d4bd70dbca1b2fa8b