Summary:
The `EmitterSubscription.remove()` method was previously calling `this.subscriber.removeSubscription(this)` directly, bypassing the mechanism in `NativeEventEmitter` that keeps track of the number of subscriptions.
This meant that native event modules (subclasses of `RCTEventEmitter`) would keep sending events even after all the listeners had been removed. This wasn't a huge overhead, since these modules are singletons and only send one message over the bridge per event, regardless of the number of listeners, but it's still undesirable.
This fixes the problem by routing the `EmitterSubscription.remove()` method through the `EventEmitter` so that `NativeEventEmitter` can apply the additional native calls.
I've also improved the architecture so that each `NativeEventEmitter` uses its own `EventEmitter`, but they currently all still share the same `EventSubscriptionVendor` so that legacy code which registers events via `RCTDeviceEventEmitter` still works.
Reviewed By: vjeux
Differential Revision: D3292361
fbshipit-source-id: d60e881d50351523d2112473703bea826641cdef
516bf7bd94