Summary: This implements `EventHandlers` abstract class (aka "Events Guy") which encapsulates `eventDispatcher` and `instanceHandle` (and ownership of future `eventTarget`), all of this as part of existing {ShadowNode + Props + LayoutMetrics + LocalData + Descriptor + (and now) EventHandlers} infra. (We don't plan to add anything else to this model. Ever.)
Reviewed By: fkgozali
Differential Revision: D8053351
fbshipit-source-id: 1dd9ccbcbe5a2eb284b59ea351dc8beca645e8bf
2a3025da97