Handle cancelled All Apps transitions.

Previously if this was cancelled mid-animation, the background
panel could get stuck on the screen.

To reproduce this consistently, I automatically cancelled the
animation 50ms after it started. Here's how that looked:
 - Before: https://drive.google.com/file/d/1tiW7mFehk3of-qYe1_HBZSmJLjU4VVmv/view?usp=drive_link&resourcekey=0-tH7YccjcNuXe8qKaL_8H2w
 - After: https://drive.google.com/file/d/1XpRy8FusaSZkJTGa7FHhUuqMUHkeI_UA/view?usp=drive_link&resourcekey=0-gvoyXE3puv7lW78kgKLEng

Fix: 383731295
Test: Manual
Flag: EXEMPT bugfix
Change-Id: I9423b75800a002419a56a3fe3f93b2e7350aeaa0