1.2 Extract IRemoteAnimationFinishedCallback from the impl.

This removes the first half of the legacy API from the common impl
by wrapping it in a generic runnable.

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: Iddf7eb7bfa631b8c4370d40ca3acb9e951bf890f