This CL introduces BackProgressAnimator which runs in app's main thread.
It receives target progress values from SysUI and drives the actual
progress value passed to the app with a high stiffness, no bounce
spring.
Bug: 238475284
Test: atest WindowOnBackDispatcherTest
Test: atest BackAnimationControllerTest
Test: atest TouchTrackerTest
Change-Id: Ib0d3ebe43929c405b10681000fb4e7ef8bccce34