1.1 Split AnimationDelegate between a wrapper and an impl.

This is setting up for a common internal implementation with two
distinct wrappers, one using the Shell API and one using the
legacy one.

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: I04974f72f3d9a4d4d57a334cd59f0a98a7b2c45a