Use native module passed to NativeEventEmitter on Android

Summary:
## Context

In native modules implementing an event emitter, we can wait for JS to subscribe to an event before making the subscription to the right native API in the native module. This is only supported on iOS at the moment and we want to support it on Android too, so we can manage resources more efficiently and avoid custom code to do this on Android, like this:

https://www.internalfb.com/intern/diffusion/FBS/browse/master/xplat/js/RKJSModules/public/Dating/Profile/ProfileView/ProfileGemstoneProfileView.js?commit=165ad219e6bf&lines=302-304

The way this works now is by creating instances of `NativeEventEmitter`, where we pass a reference to the native module that needs to be notified when there are new subscriptions. We have explicit code to ignore this native modules in all platforms except for iOS:
https://www.internalfb.com/intern/diffusion/FBS/browsefile/master/xplat/js/react-native-github/Libraries/EventEmitter/NativeEventEmitter.js?commit=5a1e671453465e844dd851c458cb2467a2db5d03&lines=44-52

## Changes

This removes the check for iOS from `NativeEventEmitter` so we also try to use the native module to notify subscriptions on Android.

We have migrated all existing code passing a native module to `NativeEventEmtiter` to only pass it on iOS, so we don't change this behavior in existing code.

Any other existing code using this API is most likely fine too. It didn't work before so the expectation is that the native module wouldn't be implemented on Android anyway.

Changelog: [Android][Changed] - Modified `NativeEventEmitter` to also use the passed native module to report subscriptions on Android

Reviewed By: yungsters

Differential Revision: D27500994

fbshipit-source-id: ef82da04020fb08cd0ea4f1cfffd1da6453ab0b9

Libraries/EventEmitter/NativeEventEmitter.js

@@ -18,12 +18,10 @@ import Platform from '../Utilities/Platform';
 import RCTDeviceEventEmitter from './RCTDeviceEventEmitter';
 import invariant from 'invariant';
 
-type NativeModule = $ReadOnly<
-  interface {
-    addListener: (eventType: string) => void,
-    removeListeners: (count: number) => void,
-  },
->;
+interface NativeModule {
+  addListener(eventType: string): void;
+  removeListeners(count: number): void;
+}
 
 export type {EventSubscription};
 
@@ -47,8 +45,8 @@ export default class NativeEventEmitter<TEventToArgsMap: {...}>
         nativeModule != null,
         '`new NativeEventEmitter()` requires a non-null argument.',
       );
-      this._nativeModule = nativeModule;
     }
+    this._nativeModule = nativeModule;
   }
 
   addListener<TEvent: $Keys<TEventToArgsMap>>(