Allow KAutostart to be constructed using an absolute path

Summary:
If we already have found the service we need to autostart, it's a waste
to strip the full path to have it located again.

Test Plan: tests still pass, works with my plasma-workspace patch

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22387