Handle spawning kinfocenter with a full path

Summary:
/usr/bin/kinfocenter doesn't match "kinfocenter"

it also cannot be using as the binaryName in the rest of this method.

Test Plan:
Launched
systemsettings5
kinfocenter
/opt/kde5/bin/kinfocenter

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: pino, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27137