use the decoded url for launcherPosition

Summary:
launchersOrder and the actual url data can be different:
in case of preferred:/ urls launcherorder will have that for
(for serialization) and the actual appdata url will be decoded as for
instance Applications:firefox.desktop.
this url is the one that needs to be compared to know the actual
launcher order

BUG:418483

Test Plan: firefox stays in first position when pinned as preferred://browser

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28632