Fix hardcoded notch marginTop in RNTester (#55132)

Summary:

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

Author: sbuggay

packages/rn-tester/js/components/RNTTitleBar.js

@@ -98,7 +98,7 @@ const styles = StyleSheet.create({
   header: {
     height: 40,
     flexDirection: 'row',
-    marginTop: Platform.OS === 'ios' ? 50 : 0,
+    marginTop: Platform.OS === 'ios' && !Platform.isTV ? 50 : 0,
   },
   headerCenter: {
     flex: 1,