fix(iOS): remove alert's window when call to hide.

Resolves this issue: facebook#32304.

Without this change, calling to hide an alert, leaves a `UIWindow`
that blocks user interactions with the screen.

The correct way to remove a `UIWindow` in iOS is to set its
hidden property to `YES`.

Also, it is required to remove all references to the window (the
associated `windowScene` for example) and ARC will automatically
free this `UIWindow`.

React/CoreModules/RCTAlertController.m

@@ -35,6 +35,8 @@ - (void)show:(BOOL)animated completion:(void (^)(void))completion
 
 - (void)hide
 {
+  [_alertWindow setHidden: YES];
+  _alertWindow.windowScene = nil;
   _alertWindow = nil;
 }