Fix association-with-derived-mimetype again.

871cccc8 made it impossible to re-order file type associations.
7f42bf25 fixed that, but changed the value of KService::mimeTypes(), which
broke okular. This new fix works the same way, but only inside kbuildsycoca
when it processes the mimetypes. The value of KService::mimeTypes() is now
restored to be exactly what's in the desktop file.

CCBUG: 321706
FIXED-IN: 4.11.3