Fixed flickering when swiping away last notification

Because the last section includes the shelf, it needs
to use the final position of the shelf instead of the
current position for the minimum bottom location.
Otherwise the background cancels the animation
immediately and things go wrong.

Fixes: 127807981
Test: atest SystemUITests
Change-Id: Ide3e8e94129bd77cebf68ff053275f9177db69f0