Use mimeapps.list "Default Applications" to prepend the service to the offers for this mimetype.
So that `KMimeTypeTrader::preferredService` will respect the setting.
Use `KMimeTypeTrader::preferredService` to replace usage of KEmailSettings / emaildefaults.
Specs:
https://specifications.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html
https://specifications.freedesktop.org/mime-apps-spec/mime-apps-spec-latest.html
CCBUG: 403499