Properly mark root shadow node as mounting to a View

Summary: RootFlatShadowNode always mounts to a (top-level) View but was never marked as one that mounts to a View. This diff fixes it. Nothing in the code relied on it being marked as mounting to a View, so it worked fine. A follow up path that implements UIImplementation.measure() for nodes that don't mount to a View goes up until it finds first mounting node, and it was crashing with NPE because it tried to access RootFlatShadowNode's parent (and there isn't one).

Reviewed By: sriramramani

Differential Revision: D2800818

ReactAndroid/src/main/java/com/facebook/react/flat/FlatRootShadowNode.java

@@ -14,6 +14,11 @@ package com.facebook.react.flat;
  */
 /* package */ final class FlatRootShadowNode extends FlatShadowNode {
 
+  /* package */ FlatRootShadowNode() {
+    forceMountToView();
+    signalBackingViewIsCreated();
+  }
+
   private boolean mIsUpdated;
 
   /**