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.

Change-Id: I3c793c3520fc6f9a18d5a3abf1fe0df803c5e580

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