Interface translations override config translations on update

Problem/Motivation

When executing locale updates any existing config translations are overridden by interface translations if the source string is the same. Expectation is that if you edit a config translation it shouldn't be overridden by locale updates.

Steps to reproduce

1. I have some country names translated in locale E.g. United States = アメリカ in Japanese.
2. We display country names in a language selector. That information is stored in simple config.
3. Feedback from client was that country names should be displayed in their own language in the language selector. Translations should still exist in other contexts though.
4. Put United States for the translation of United States in the Japanese config for the language selector. You could even delete the whole config file.
5. Re-import locale like so: drush locale:import --type customized --override customized ja /app/translations/ja.po
6. Export your config. You will notice that the config has been overridden with アメリカ

Proposed resolution

Determine if this is a bug or if a new option needs to be added to config to stop it getting overridden.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet