power: Add panel auto-brightness configuration

* Some panel hardware requires that we twiddle some knobs in sysfs to
   enable proper display calibration (gamma, etc) when auto-brightness
   is enabled. Add a configuration value to handle this.

Conflicts:
core/res/res/values/symbols.xml

Change-Id: I149d885ed9b0385f41d3212304b570e7c816d46b