fix: color scheme in appearance state after setting it to unspecified (#54993)

Summary:
Fixed the value of the color scheme in the appearance state after setting it to unspecified when state updates. The issue is described here #54959. This is a regression from #53397 and should be reproducible from 0.82 and higher. The `useColorScheme()` hook will return "unspecified" when you change the appearance from system color (dark or light) to system which breaks many apps using the value to determine whether to use dark/light mode colors.

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[GENERAL] [FIXED] - Fix color scheme in appearance state after setting it to unspecified

Pull Request resolved: #54993

Test Plan:
1. Run the reproducer app in the issue - use dark mode when launching the app, press "change to dark" and then "change to system". The hook will return "unspecified" although the correct behavior should be dark.
2. Apply the changes in this PR.
3. The correct behavior should be seen.

Reviewed By: cipolleschi

Differential Revision: D92391594

Pulled By: cortinico

fbshipit-source-id: 3c0c471069f2eb4681bd53abf67957bcaa04e7b4

Author: ismarbesic

packages/react-native/Libraries/Utilities/Appearance.js

@@ -99,7 +99,12 @@ export function setColorScheme(colorScheme: ColorSchemeName): void {
   if (NativeAppearance != null) {
     NativeAppearance.setColorScheme(colorScheme);
     state.appearance = {
-      colorScheme,
+      // When setting to 'unspecified', get the actual system color scheme.
+      // Fall back to the passed value if getColorScheme() returns null.
+      colorScheme:
+        colorScheme === 'unspecified'
+          ? (NativeAppearance.getColorScheme() ?? colorScheme)
+          : colorScheme,
     };
   }
 }