Summary:
`forkEvent` is generally used to intercept an existing listener and add a new js listener to it, considering the original listener can be null/js/Animated.event(). I added tests to ensure the 3 cases of the implementation are all covered.
Pull Request resolved: https://github.com/facebook/react-native/pull/20111
Differential Revision: D8817500
Pulled By: hramos
fbshipit-source-id: 1a20b6f73e2d47bbefccd31378764909a45e89bb
f0d35904c9