DeskClock: Snooze: Mimic dismiss reveal

* Right now, the dismiss animation starts and ends with the accent
  color,  which is a very light yellow
* Especially when you use "snooze" as in "I want to continue sleeping a
  little more", the whole screen turning bright isn't helping
* Mimic the behavior of the dismiss button, which starts and ends with
  a shade of gray

Change-Id: I7d0e387a247015d6e0eb5cfab7b393d9b48996e3