DeskClock: Come to the dark side...

... we have cookies

* Move minSdkVersion to 24 so the notification actions don't use the
  icons anymore and we can just tint the icons black to fit onto the new
  accent color used e.g. for the FABs
* Move accent color to an own color name which is defined differently
  for values and values-night
* Background: Keep blue for light theme but use grey for night one

Change-Id: Ib14044fc9c6de15453891638581a7e3f4d10c6c5