iOS Surface: properly reset some internal states when JS reloads

Summary:
There are a few important states that didn't reset correctly when reloading JS:
* the RCTSurfaceStage was stuck at all bits enabled, hence no further stage change happened (even though the state "reset" to `RCTSurfaceStageBridgeDidLoad`)
* the RCTSurfaceView didn't get recreated, because the _view ivar was never cleared
* similarly, the _touchHandler ivar attached to the _view was never re-setup --> all touches after JS reload were dropped before this diff

Reviewed By: mmmulani

Differential Revision: D7178038

fbshipit-source-id: ba49bc205f8bf43842471b7ab748cef8549ea212

React/Base/Surface/RCTSurface.mm

@@ -186,6 +186,10 @@ - (void)handleBridgeWillLoadJavaScriptNotification:(NSNotification *)notificatio
 {
   RCTAssertMainQueue();
 
+  // Reset states because the bridge is reloading. This is similar to initialization phase.
+  _stage = RCTSurfaceStageSurfaceDidInitialize;
+  _view = nil;
+  _touchHandler = nil;
   [self _setStage:RCTSurfaceStageBridgeDidLoad];
 }