Fix OnBackAnimationCallback not receiving updates

`mProgressAnimator` in `WindowOnBackInvokedDispatcher` is static, so we must not reset it when the `WindowOnBackInvokedDispatcher` gets detached, or otherwise we risk interfering animations that belong to another `WindowOnBackInvokedDispatcher`.

Bug: 316016446
Flag: NONE
Test: atest WindowOnBackInvokedDispatcherTest
Test: Manual, i.e. verifying that back animations do not get stuck in the middle of the animation
Change-Id: I4da0179e5e5d02f5ce6fb8af6dc29b31a2101c32