Extract getViewportOffset out RootView

Summary:
This diff moves the method getViewportOffset out of ReactRootView. This is necessary to avoid Fabric to depend from paper.

changelog: [internal] internal

Reviewed By: JoshuaGross

Differential Revision: D26716901

fbshipit-source-id: cec67c24860a776fb361d7cda08d3142e1214c8c

ReactAndroid/src/main/java/com/facebook/react/fabric/Binding.java

@@ -91,6 +91,7 @@ public class Binding {
         jsMessageQueueThread,
         componentFactory,
         reactNativeConfig);
+
     setPixelDensity(PixelUtil.getDisplayMetricDensity());
   }