fixed polyfill for BackAndroid

Summary:
`BackAndroid.addEventListener()` returns a subscription object with a `remove()` function in android. Before this fix, the iOS equivalent doesn't return anything, which means, if there's a component doing something like this, it would redbox:

```
componentWillMount() {
  this._subscription = BackAndroid.addEventListener('hardwareBackPress', () => {...});
}

componentWillUnmount() {
  this._subscription.remove(); // --> redbox in iOS before this fix
}
```

Differential Revision: D3790480

fbshipit-source-id: 1e607171bf2892a6b64977c4fd052c5df0bc4a0d

Libraries/Utilities/BackAndroid.ios.js

@@ -17,7 +17,11 @@ function emptyFunction() {}
 
 const BackAndroid = {
   exitApp: emptyFunction,
-  addEventListener: emptyFunction,
+  addEventListener() {
+    return {
+      remove: emptyFunction,
+    };
+  },
   removeEventListener: emptyFunction,
 };