Fixed the animations on the notification scrims

When pulling down on a heads up notification the scrim would
fade in from 0 instead of fading to the new position.
The old behavior didn't work if the heads up notifications were
big, so we had to change it in order to always have a scrim.

Change-Id: I9ae387b75136c9240d863e06a8649c85b3c6c9dc