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

JIRA:CYAN-612
Change-Id: I64274280872b9fe4f6fdc368d654aca81e0a4e0c