Animate thumbnail dim after quick scrub settles

We now animate the dim alpha at the same time as the task icon.

I also clarified the dim alpha by inverting it where it is used in
TaskThumbnailView. Otherwise, we have to pass 1 - dimAlpha everywhere,
which is confusing and verbose.

Bug: 76170043
Change-Id: If8c01300993a0e5543b0650d44daaa143592b96e
(cherry picked from commit 291bfd9c)