Use atomic pointer for animationDelegate_ to prevent race during teardown

Summary:
Changelog: [internal]

[This assumption](https://fburl.com/diffusion/yeqtvxru) in `~Scheduler` might not be completely correct. There can be a race between JS thread and main thread. With main thread settings animationDelegate_ to nullptr and JS thread reading the value. Even though they always happen one after the other, JS thread can have the value of animationDelegate_ in its cache line and changing it from main thread might not invalidate it right away.

Reviewed By: JoshuaGross

Differential Revision: D29237290

fbshipit-source-id: 6cb898caf7c225cf8162e7560921b563dec514b1