Migrate fabric.mounting.mountitems subset to Kotlin (#44440)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44440

# Changelog:
[Internal] -

As in the title, note that there are more files there to migrate, will come separately, to make reviewing easier.

Reviewed By: javache

Differential Revision: D57046953

fbshipit-source-id: e45316da1ed9caaa4daafa96dfabfd374926bd73