Rely upon QSysInfo to retrieve the system details

Summary:
QSysInfo works on both unix like and windows OS's.
Better to use it to gather information about the system.

Test Plan:
Ran ctest, tests executed successfully

kcmshell5 --useragent

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: pino, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20626