sdk: Stop using lerp for night/day mode transitions

* This makes it clear what the desired behavior is
* In order to make the logic more clear, also start
  scaling into night mode before civil sunset so that
  we finish by civil sunset -- this makes sure that
  by the time there's no light out for practical
  purposes, we are in night mode
* While we're at it, add a few comments about what
  each return is supposed to do, for the future

Change-Id: I4ec0f8eec6b0129a37e99f9e3a2b3f124aeeb4f2