Summary:
As we did in the previous diff, here we implemented `EventEmitter`'s ownership model as a `shared_ptr`. This change fixes problem with leaking `WeakObject`s which happens on hot-reload.
So, in short:
* `EventTargetWrapper` object owns `jsi::WeakObject` that can be converted to actual `jsi::Object` that represent event target in JavaScript realm;
* `EventTargetWrapper` and `jsi::WeakObject` objects must be deallocated as soon as native part does not need them anymore;
* `EventEmitter` objects retain `EventTarget` objects;
* `EventEmitter` can loose event target object in case if assosiated `ShadowNode` got unmounted (not deallocated); in this case `EventEmitter` is loosing possibility to dispatch event even if some mounting-layer code is still retaining it.
Reviewed By: mdvacca
Differential Revision: D9762755
fbshipit-source-id: 96e989767a32914db9f4627fce51b044c71f257a
c25d5948a5