1.4 Introduce AnimatedWindow to make the impl API-agnostic.

Trying to make each of the CLs as small as possible to keep them
digestible and low risk. For the refactor plan see
go/animlib-shell-refactor-plan.

Bug: 397180418
Flag: EXEMPT simple refactor
Test: atest ActivityTransitionAnimatorTest + manual
Change-Id: I12326dfae4486d2640496a52861412b751e39b2b