Don't fetch device statistics upon construction

Summary:
To get proper data we'll have to change the refresh rate, so just wait until
it's done and we'll get all the data.

See T10958

Test Plan: tests pass, the plasmoid still shows the statistics when asked to.

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D21486