Hotfix privacy manifest aggregation script (#44390)

Summary:
As pointed out by liamjones here:
https://github.com/facebook/react-native/pull/44214#discussion_r1587755403

The original PR did introduce a bug in the `find/first` check, but in my testing, we do need to look at `group.name`, so let's make sure we check both.

This also makes it play nice with an existing file even if it is added to a different directory, by appending to it instead of forcing it to exist in the main group.

## Changelog:

[IOS] [FIXED] - Fix privacy aggregation

Pull Request resolved: https://github.com/facebook/react-native/pull/44390

Test Plan: Tested on rn-tester

Reviewed By: cipolleschi

Differential Revision: D56893594

Pulled By: philIip

fbshipit-source-id: b92589bc2bed9d07e9af20c56a8b9f6c61d864f0