Summary:
This PR fixes a few issues with the Appearance API (as noted here #28823).

1. For the Appearance API to work correctly on Android you need to call `AppearanceModule.onConfigurationChanged` when the current Activity goes through a configuration change. This was being called in the RNTester app but not in `ReactActivity` so it meant the Appearance API wouldn't work for Android in newly generated RN projects (or ones upgraded to the latest version of RN).

2. The Appearance API wasn't working correctly for brownfield scenarios on Android. It's possible to force an app light or dark natively on Android by calling `AppCompatDelegate.setDefaultNightMode()`. The Appearance API wasn't picking up changes from this function because it was using the Application context instead of the current Activity context.

3. The Appearance API wasn't working correctly for brownfield scenarios on iOS. Just like on Android its possible to force an app light or dark natively by setting `window.overrideUserInterfaceStyle`. The Appearance API didn't work with this override because we were overwriting `_currentColorScheme` back to default as soon as we set it.

## Changelog

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### Fixed

#28823

* [Android] [Fixed] - Appearance API now works on Android
* [Android] [Fixed] - Appearance API now works correctly when calling `AppCompatDelegate.setDefaultNightMode()`
* [iOS] [Fixed] - Appearance API now works correctly when setting `window.overrideUserInterfaceStyle`

Pull Request resolved: #29106

Test Plan: Ran RNTester on iOS and Android and verified the Appearance examples still worked [correctly.](url)

Reviewed By: hramos

Differential Revision: D31284331

Pulled By: sota000

fbshipit-source-id: 45bbe33983e506eb177d596d33ddf15f846708fd