Dispatch events even when there are no listeners

Summary:
## Rationale
For every 1 call to RCTNetworking.sendRequest, we execute 6 calls to RCTNetworking.addListener. This is followed by at least one call to RCTNetworking.removeListeners. Aside from incrementing and decrementing the `_listeners` integer, these two methods accomplish nothing else: RCTNetworking doesn't implement the `startObserving` and `stopObserving` methods.

This diff makes RCTEventEmitter dispatch events without looking at the listeners integer. In the future, this will allow us to stop making these ~8 unnecessary NativeModule calls for every Network request we send.

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D24272560

fbshipit-source-id: 7996eba5abfa4669a89c43a3ffa536c0faa214a8

Libraries/Network/RCTNetworking.mm

@@ -159,7 +159,7 @@ RCT_EXPORT_MODULE()
 
 - (instancetype)initWithHandlersProvider:(NSArray<id<RCTURLRequestHandler>> * (^)(void))getHandlers
 {
-  if (self = [super init]) {
+  if (self = [super initWithDisabledObservation]) {
     _handlersProvider = getHandlers;
   }
   return self;