Implement eager initialization of Fabric

Summary:
This diff implements the eager initialization of fabric based on the param created in previous diffs

changelog: [Internal] Internal change in Fabric

Reviewed By: JoshuaGross

Differential Revision: D21574815

fbshipit-source-id: 1dfd2611ce8c8529ce5f6a7a8c48f8bee19be256

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

@@ -1303,6 +1303,9 @@ public class ReactInstanceManager {
         }
       }
     }
+    if (ReactFeatureFlags.eagerInitializeFabric) {
+      catalystInstance.getJSIModule(JSIModuleType.UIManager);
+    }
     if (mBridgeIdleDebugListener != null) {
       catalystInstance.addBridgeIdleDebugListener(mBridgeIdleDebugListener);
     }