iOS: Fix RCTDevLoadingView not showing up with UIScene (#27916)

Summary:
Attaches the loading view to a `UIScene` in apps using the new Scenes API (iOS 13+).

## Changelog

[iOS] [Fixed] - Fix RCTDevLoadingView not showing up with UIScene
Pull Request resolved: #27916

Test Plan:
Create a new app based on UIScene and integrate React Native.

| Before | After  |
|:------:|:------:|
 | <img width="453" alt="image" src="https://user-images.githubusercontent.com/4123478/73485717-eccfe800-43a3-11ea-96fd-f7077a348345.png"> | <img width="453" alt="image" src="https://user-images.githubusercontent.com/4123478/73485749-f9ecd700-43a3-11ea-8a18-2e2185e62e78.png"> |

Reviewed By: PeteTheHeat

Differential Revision: D20104655

Pulled By: hramos

fbshipit-source-id: 86c1902c00f791fd23f75ea7562b44c92e719c5e

React/DevSupport/RCTDevLoadingView.m

@@ -94,6 +94,11 @@ - (void)setBridge:(RCTBridge *)bridge
     self->_label.textColor = color;
     self->_window.backgroundColor = backgroundColor;
     self->_window.hidden = NO;
+
+    if (@available(iOS 13.0, *)) {
+      id scene = [[RCTSharedApplication() valueForKey:@"connectedScenes"] anyObject];
+      [self->_window setValue:scene forKey:@"windowScene"];
+    }
   });
 }