Fixes background animation on dismiss.

This change modifies the logic in
NotificationStackScrollLayout.updateFirstAndLastBackgroundViews() which
determines if the first or last visible notification row has changed.
The "old" values were being read after the changes were applied, meaning
that the change was never detected, so an animation was never started.

Test: manually
Change-Id: I29869f5b0f78fd81a17d2c3dbe18c067f94fdb5e
Fixes: 119792278