refactor: improve useColorScheme subscription efficiency (#38001)

Summary:
motivation: re-rendering `useColorScheme()` hook with what is on the main branch means there is a subscribe + unsubscribe dance happening.

related docs: https://react.dev/reference/react/useSyncExternalStore#my-subscribe-function-gets-called-after-every-re-render

## Changelog:

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[INTERNAL] [CHANGED] - improve useColorScheme subscription efficiency

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Pull Request resolved: https://github.com/facebook/react-native/pull/38001

Test Plan: tested locally in an app

Reviewed By: rshest

Differential Revision: D49008053

Pulled By: javache

fbshipit-source-id: f8a9fc8950277e2bae8bd8774bf21312a67d46d5

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

@@ -15,12 +15,11 @@ import type {ColorSchemeName} from './NativeAppearance';
 import Appearance from './Appearance';
 import {useSyncExternalStore} from 'react';
 
+const subscribe = (onStoreChange: () => void) => {
+  const appearanceSubscription = Appearance.addChangeListener(onStoreChange);
+  return () => appearanceSubscription.remove();
+};
+
 export default function useColorScheme(): ?ColorSchemeName {
-  return useSyncExternalStore(
-    callback => {
-      const appearanceSubscription = Appearance.addChangeListener(callback);
-      return () => appearanceSubscription.remove();
-    },
-    () => Appearance.getColorScheme(),
-  );
+  return useSyncExternalStore(subscribe, Appearance.getColorScheme);
 }