Fix hardcoded notch marginTop in RNTester (#55132)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55132

RNTester doesnt use safe areas and instead hardcodes the margin top based on Platform.OS. Issue is that AppleTV returns `ios` so there is a gap since there is no notch. Instead of adding a new Metro platform, just using the deviceIdiom instead for now.

Changelog: [internal]

Reviewed By: Abbondanzo

Differential Revision: D90514023

fbshipit-source-id: 9d41017ccbfd0376a304cae3177d75e67fe14d3f