Fix ReactInstanceManager to initialize the UIManager (facebook#41594)

Summary:
Adding `initialize()` to FabricUIManager just as was done by JSIModule

Without this change switching to UIManagerProvider would cause the UI to be Frozen and the events not correctly registered.

Pull Request resolved: facebook#41594

Reviewed By: javache

Differential Revision: D51456979

fbshipit-source-id: 8d97533340a88ec6bb2bf0f257b6acfaa59da471

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -1408,7 +1408,9 @@ private ReactApplicationContext createReactContext(
     }
     if (ReactFeatureFlags.enableFabricRenderer) {
       if (mUIManagerProvider != null) {
-        catalystInstance.setFabricUIManager(mUIManagerProvider.createUIManager(reactContext));
+        UIManager uiManager = mUIManagerProvider.createUIManager(reactContext);
+        uiManager.initialize();
+        catalystInstance.setFabricUIManager(uiManager);
       } else {
         catalystInstance.getJSIModule(JSIModuleType.UIManager);
       }