Animated: Store Listeners w/ Map

[yungsters]

Summary: Currently, AnimatedNode.prototype.getListeners creates an array with Object.keys() to determine the number of listeners.

This is a relatively hot code path for animation-intensive user interfaces. Although Object.keys() is fast, every unnecessary memory allocation is an unnecessary opportunity to create garbage that requires collection.

Using an object as a dictionary performs worst than using a Map anyway, so this switches AnimatedNode to use a Map.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D62267352

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D62267352