Fix retrieving current appearance in multi-window apps (facebook#42231)

Summary:
This PR resolves issues with retrieving appearance in multi-window apps by calling `RCTKeyWindow()` instead of retrieving the AppDelegate window property. It also does small optimization in the RCTAlertController.

[IOS] [FIXED] - Fix retrieving current appearance in multi-window apps

Pull Request resolved: facebook#42231

Test Plan: CI Green, it should work the same as before

Reviewed By: NickGerleman

Differential Revision: D52802756

Pulled By: cipolleschi

fbshipit-source-id: 60b5f7045f41be19caae5102f0dc321d4ecdcd2f

packages/react-native/React/CoreModules/RCTAlertController.mm

@@ -38,9 +38,8 @@ - (void)show:(BOOL)animated completion:(void (^)(void))completion
 {
   UIUserInterfaceStyle style = self.overrideUserInterfaceStyle;
   if (style == UIUserInterfaceStyleUnspecified) {
-    style = RCTSharedApplication().delegate.window.overrideUserInterfaceStyle
-        ? RCTSharedApplication().delegate.window.overrideUserInterfaceStyle
-        : UIUserInterfaceStyleUnspecified;
+    UIUserInterfaceStyle overriddenStyle = RCTKeyWindow().overrideUserInterfaceStyle;
+    style = overriddenStyle ? overriddenStyle : UIUserInterfaceStyleUnspecified;
   }
 
   self.overrideUserInterfaceStyle = style;

packages/react-native/React/CoreModules/RCTAppearance.mm

@@ -96,7 +96,7 @@ - (instancetype)init
 {
   if ((self = [super init])) {
 #if !TARGET_OS_OSX // [macOS]
-    UITraitCollection *traitCollection = RCTSharedApplication().delegate.window.traitCollection;
+    UITraitCollection *traitCollection = RCTKeyWindow().traitCollection;
     _currentColorScheme = RCTColorSchemePreference(traitCollection);
 #else // [macOS
 	NSAppearance *appearance = RCTSharedApplication().appearance;