Expose in public interface

Summary:
Allows you to do:
```
var { RecyclerViewBackedScrollView } = require('react-native')
```

Rather than:
```
var RecyclerViewBackedScrollView = require('react-native/Libraries/Components/ScrollView/RecyclerViewBackedScrollView')
```

Also...

- Export `ScrollView` by default rather than `UnimplementedView` for `RecyclerViewBackedScrollView` on iOS -- this makes it easier on the user, so you don't have to always do a conditional for: `if IOS then use ScrollView else use RecyclerViewBackedScrollView`. I can't think of a case where this would lead to undesirable behaviour.
- Add `RecyclerViewBackedScrollView` to `MainReactPackage`
- Fix an issue with `MapView` that threw a red-screen when trying to access constants on Android because there is no `MapView` in open source and MapView.js doesn't have a platform extension.
Closes https://github.com/facebook/react-native/pull/4514

Reviewed By: svcscm

Differential Revision: D2753466

Pulled By: mkonicek

fb-gh-sync-id: 0b6e2133975c911d5117e7531cb9093faf314c52

ReactAndroid/src/main/java/com/facebook/react/shell/MainReactPackage.java

@@ -30,6 +30,7 @@ import com.facebook.react.uimanager.ViewManager;
 import com.facebook.react.views.drawer.ReactDrawerLayoutManager;
 import com.facebook.react.views.image.ReactImageManager;
 import com.facebook.react.views.progressbar.ReactProgressBarViewManager;
+import com.facebook.react.views.recyclerview.RecyclerViewBackedScrollViewManager;
 import com.facebook.react.views.scroll.ReactHorizontalScrollViewManager;
 import com.facebook.react.views.scroll.ReactScrollViewManager;
 import com.facebook.react.views.switchview.ReactSwitchManager;
@@ -78,6 +79,7 @@ public class MainReactPackage implements ReactPackage {
       new ReactImageManager(),
       new ReactProgressBarViewManager(),
       new ReactRawTextManager(),
+      new RecyclerViewBackedScrollViewManager(),
       new ReactScrollViewManager(),
       new ReactSwitchManager(),
       new ReactTextInputManager(),