Sync BackAnimationControllerTest on master to the version on tm-dev.

The file has diverged quite a bit and is causing merge headaches for ag/18961750.

The updated version is copied verbatim from ToT tm-dev libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/back/BackAnimationControllerTest.java.

Test: atest BackAnimationControllerTest
Bug: 236431156

Change-Id: Ia6fdbef2236c0f49ff66b5e9d48de0db0599f8db