Consider using LanguageManager's API for managing config translations

Problem/Motivation

In 📌 Provide tests Active , to be able to create a successful plugin test for \Drupal\babel\Plugin\Babel\TranslationType\Config, I used LanguageConfigOverrides (through language manager) to see the actual results of a translation update / addition.

This was not just much simpler, but I had to change the preexisting approach also because invalidating the corresponding caches and triggering the right events were also missed.

Scope of this task is to also change the approach inside ::getStrings and ::getString methods and rely on public LanguageManager methods instead of reading config storage directly.

Remaining tasks

TBD.

User interface changes

Nothing.

API changes

No API change expected inside or outside Babel.

Data model changes

Nothing.