lights: Automatically generate an LED color for notifications

 * When an app doesn't specify a color for the LED, as is the usual
   case, we currently just show the default color. This is boring.
   It's also a pain in the ass to manually map all your apps to
   colors.
 * Use our new helpers to generate this color based on the application's
   icon color. Keep a cache to avoid wasting cycles.

Change-Id: I7288e52499819a6a6c75ed9d9ba7cfa1b13c5669