Summary:
Since `dealloc` can be called from any thread, this would result `stopObserving` being called on a different thread/queue than the specified `methodQueue`. We specifically encountered this issue with a module needing the main queue having its `stopObserving` called on a background queue.
Changelog:
[iOS][Fixed] - Call [RCTEventEmitter stopObserving] on specified method queue
Reviewed By: RSNara
Differential Revision: D23821741
fbshipit-source-id: 693c3be6876f863da6dd214a829af2cc13a09c3f