Pass fabric flag from native to JS

Reviewed By: mdvacca

Differential Revision: D7373722

fbshipit-source-id: 3cd051f38677900693f3da797effa11f9161df37

ReactAndroid/src/main/java/com/facebook/react/ReactRootView.java

@@ -490,6 +490,9 @@ public class ReactRootView extends SizeMonitoringFrameLayout
         if (appProperties != null) {
           appParams.putMap("initialProps", Arguments.fromBundle(appProperties));
         }
+        if (isFabric()) {
+          appParams.putBoolean("fabric", true);
+        }
 
         mShouldLogContentAppeared = true;