Revert "Revert Change I5ec0e757: Don't enable the light sensor if it's not being used"

This reverts commit 8b1c67ae.
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