Add ContentView after TopBar (#871)

closes #851

android/app/src/main/java/com/reactnativenavigation/react/NavigationReactGateway.java

 
 import javax.annotation.Nullable;
 
-//import android.view.View;
-
 public class NavigationReactGateway implements ReactGateway {
 
     private final ReactNativeHost host;

android/app/src/main/java/com/reactnativenavigation/screens/CollapsingSingleScreen.java

         contentView = new CollapsingContentView(getContext(), screenParams.screenId, screenParams.navigationParams);
         setViewMeasurer();
         setupCollapseDetection((CollapsingTopBar) topBar);
-        addView(contentView, createLayoutParams());
+        post(new Runnable() {
+            @Override
+            public void run() {
+                addView(contentView, createLayoutParams());
+            }
+        });
     }
 
     private void setViewMeasurer() {