Create binding for unmountComponentAtNode in bridgeless mode

Summary: Right now we register ReactFabric as a callable module with the bridge so that we can call `ReactFabric.unmountComponentAtNode` in `ReactInstanceManager.detachViewFromInstance`. In bridgeless mode we don't have callable modules, so I'm just setting a global variable that can be called from C++ instead. Using this in a new `unmount` method in FabricUIManager.

Reviewed By: shergin, mdvacca

Differential Revision: D16273720

fbshipit-source-id: 95edb16da6566113a58babda3ebdf0fc4e39f8b0

ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -182,6 +182,10 @@ public class FabricUIManager implements UIManager, LifecycleEventListener {
     mEventDispatcher.dispatchAllEvents();
   }
 
+  public void stopSurface(int surfaceID) {
+    mBinding.stopSurface(surfaceID);
+  }
+
   @Override
   public void initialize() {
     mEventDispatcher.registerEventEmitter(FABRIC, new FabricEventEmitter(this));