Dim the screen quickly in response to user activity timeout.

Reverts a previous change that made the screen dim slowly instead.
The quick transition does a better job of attracting the user's
attention to the fact that the screen is about to turn off
unless the user touches the screen.

Bug: 7386034
Change-Id: I81e4d8939f6791b96352004984a9e5b2aab79788