Fix retrieving current appearance in multi-window apps (#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.

## Changelog:

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

Pull Request resolved: #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

@@ -35,9 +35,8 @@ - (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

@@ -70,7 +70,7 @@ @implementation RCTAppearance {
 - (instancetype)init
 {
   if ((self = [super init])) {
-    UITraitCollection *traitCollection = RCTSharedApplication().delegate.window.traitCollection;
+    UITraitCollection *traitCollection = RCTKeyWindow().traitCollection;
     _currentColorScheme = RCTColorSchemePreference(traitCollection);
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(appearanceChanged:)