KCM/Component email: simplify code

Summary:
Clean up adding entries for Added Associations section of mimeapplist
Just find the application set if it is not found by the normal query.
Also add a constant and reuse one.

Reviewers: #plasma, ervin, bport, crossi

Reviewed By: ervin, bport

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27116