sdk: Split TWILIGHT_ADJUSTMENT_TIME in half

* The difference between civil and actual sun{rise,set}
  is 6 degrees of movement of the Sun in its path in
  the sky, astronomically speaking
* Since the Sun "travels" at 15 degrees/hr in the sky,
  that means the difference between civil and actual sun{rise,set}
  is about 24 minutes, temporally speaking
* In order to more closely match that temporal difference,
  the TWILIGHT_ADJUSTMENT_TIME should be about 30 minutes

Change-Id: I30a600c71ce5dd01fcb96fb4f5108f45fa493b0c