KUrlMimeData: fix handling of PreferLocalUrls

Summary: 18e4d245d3 didn't quite have the right logic after all

Test Plan: Unittest passes (but this commit proves that it doesn't cover all cases...)

Reviewers: mwolff, jtamate, markg

Reviewed By: jtamate, markg

Subscribers: markg, ngraham, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10257