[Task Manager] Strip application:// prefix before looking up service

According to Unity Launcher API documentation [1] this prefix should be there, actually, so we weren't standard-compliant here.

[1] https://wiki.ubuntu.com/Unity/LauncherAPI

CHANGELOG: Badges and progress display in task manager now works with more 3rd party applications

Differential Revision: https://phabricator.kde.org/D3416