Fixes background animation when dismissing top item in a section.

This change modifies the code that computes the target bounds for each
notification section's background to reuse the logic used to compute
the overall background's bounds.  This allows us to avoid, for example,
animating the bottom section's background downwards when dismissing the
first item in that section when that item will eventually end up exactly
where the background's top already is.

Change-Id: I3153ac47915e194b9355354593c81e84c89fc8a3
Fixes: 120873029
Test: manual