Fix modals not being dismissed properly

Improve the algorithm for determining the view controller for presentation of
RCTModalHostViewController.

The problem was that RCTModalHostViewManager always used the first responder
view controller (`[modalHostView reactViewController]`), whereas it is possible
that such controller also had presented another view controller on top of
itself (and that one could present yet another VC, and so on). So we walk
through the presentedViewController list to find the top-most presented VC.

React/Views/RCTModalHostViewManager.m

@@ -74,7 +74,19 @@ - (void)presentModalHostView:(RCTModalHostView *)modalHostView withViewControlle
   if (_presentationBlock) {
     _presentationBlock([modalHostView reactViewController], viewController, animated, completionBlock);
   } else {
-    [[modalHostView reactViewController] presentViewController:viewController animated:animated completion:completionBlock];
+    UIViewController *topViewController = [modalHostView reactViewController];
+    while (topViewController.presentedViewController != nil) {
+      if ([topViewController.presentedViewController isKindOfClass:UIAlertController.class]) {
+        // Don't present on top of UIAlertController, this will mess it up:
+        // https://stackoverflow.com/questions/27028983/uialertcontroller-is-moved-to-buggy-position-at-top-of-screen-when-it-calls-pre
+        [topViewController dismissViewControllerAnimated:animated completion:^{
+          [topViewController presentViewController:viewController animated:animated completion:completionBlock];
+        }];
+        return;
+      }
+      topViewController = topViewController.presentedViewController;
+    }
+    [topViewController presentViewController:viewController animated:animated completion:completionBlock];
   }
 }