Only apply wallpaper anim if top most app is wallpaper target

If it's not the top app it doesn't make a lot of sense to use
that animation, as it doesn't really work for these other
use-cases when Launcher is behind a translucent activity.

Fix: 78995541
Test: go/wm-smoke
Test: Open Assistant, click explore button, click back, observe
no freeze.
Change-Id: I0bc0f4e066f55c545ac2d3254b7391ec97db425f