Make DevSettingsActivity internal (#49073)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49073

This activity should be internal as it's exposed by ReactNative's Debug Manifest.
No need to have it public. I checked that there are no OSS usages of it:

https://www.google.com/url?q=https://github.com/search?type%3Dcode%26q%3DNOT%2Bis%253Afork%2BNOT%2Borg%253Afacebook%2BNOT%2Brepo%253Areact-native-tvos%252Freact-native-tvos%2BNOT%2Brepo%253Anuagoz%252Freact-native%2BNOT%2Brepo%253A2lambda123%252Freact-native%2BNOT%2Brepo%253Apvinis%252Freact-native---investigation%2BNOT%2Brepo%253Abeanchips%252Ffacebookreactnative%2BNOT%2Brepo%253AfabOnReact%252Freact-native-notes%2BNOT%2Buser%253Ahuntie%2Bcom.facebook.react.devsupport.DevSettingsActivity&sa=D&source=editors&ust=1738261498882961&usg=AOvVaw295OXKV-8dAbdsMTY5usSx

Changelog:
[Internal] [Changed] -

Reviewed By: mdvacca

Differential Revision: D68904656

fbshipit-source-id: b98b417e60a3e8ebba0c9946959ee43ea963b066