Fix DBus startup notification

Summary:
In "Fix colours not updating in systemsettings", the application name
got changed from "systemsettings5" (inferred from the binary name) to
"systemsettings", which results in a change from "org.kde.systemsettings5"
as DBus path to "org.kde.systemsettings". KLauncher uses org.kde.<binary name>
by default, so that needs to be overwritten.

Fixes the "KDEInit count not launch /usr/bin/systemsettings5" message after
closing systemsettings and likely also launching it while it's running.

Test Plan:
Opened it from dolphin and closed the window again. Only after
this patch the KDEInit error message disappeared.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: ngraham, broulik, rikmills, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8455