Overhaul auto-brightness level UI.

This changes a few things:
- Don't imply a 'bucket' type of algorithm is used by showing an ambient
  brightness range for each line
- Allow manually entering the screen brightness
- Add a preview window for the cubic spline interpolation
- Add a help text

Change-Id: Ia8747f5c17ba0323d3f9fd1433b82d3abbf70b3c