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