Save the OnBackInvokedCallback in WindowState

This CL enable OnBackInvokedCallback to be saved in a WindowState and
queried by the BackNavigationController to be returned when
ATM.startBackNavigation() is called.

The DecorView provides a PendingOnBackInvokedDispatcher that can receive
callback registration before being added to a ViewRootImpl.

Test: atest FrameworksCoreTests:BackNavigationTest
Bug: 131727607
Change-Id: I01528a22ea4a6583a56ade4eab69136d727855d0