Fix nullsafe FIXMEs for TurboModuleManager and mark nullsafe (#50370)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50370

Gone trough all the FIXMEs added in the previous diff by the nullsafe tool, marked the class as nullsafe and ensured no remaining violations.
Changelog: [Android][Fixed] Made TurboModuleManager.java nullsafe

Reviewed By: cortinico

Differential Revision: D71979605

fbshipit-source-id: b8da8f1ad12f5f6d63e7bd876d7c4912d69a4c4d