Make ProgressAnimator non-static.

This way when WindowOnBackInvokedDispatcher gets detached, it does not
risk interferring animations that belong to another
WindowOnBackInvokedDispatcher, according ag/26012753.

Bug: 316016446
Test: m -j
Test: atest OnBackInvokedCallbackGestureTest
Test: atest BackAnimationControllerTest
Test: atest WindowOnBackInvokedDispatcherTest

Change-Id: I2b386936d2fa67ac50ce9748cbb7914591e541bd