Improve auto-brightness handling

- Add a setting to control the auto-brightness algorithm responsitivity
- Make the algorithm react much faster when the ambient brightness
  increases quickly, e.g. when receiving a call in bright sunlight.

Change-Id: Ic608901c476609dde69cae982228c09567140a83
Merge "Improve auto-brightness handling" into jb-mr1

Change-Id: I5a7cd1a0fe41dad9abd0b652aabaeda63dcfa513