Move AnimationBackend initialization from Animated (#55103)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55103

This diff decouples AnimationBackend from Animated. Now the backend is intialized in the Scheduler, from where it's passed to UIManager. Animation frontends (such as Animated) can then obtain a reference to the backend, and use it to schedule animation frame updates.

# Changelog
[General] [Changed] - Moved AnimationBackend initiailzation to `Scheduler`
[General] [Added] - `AnimationChoreographer` interface with an implementation for fantom tests

Reviewed By: zeyap

Differential Revision: D89663251

fbshipit-source-id: 428a83d78ef92a89095989fe3121c578cf8d3983