Summary:
We're mutating dictionaries on one thread for bookkeeping (anytime we receive a call over the bridge) and iterating over them on the main thread each frame (in `updateAnimations`). Seems like this can all happen on the main thread without issue. Am I missing something?
**Test plan (required)**
Run UIExplorer NativeAnimated examples before and after - compare the results. Nothing should have changed.
Closes https://github.com/facebook/react-native/pull/9049
Differential Revision: D3682871
fbshipit-source-id: c6de62063e724b15b9678a9ef0290284e928b31b