Close the plasmoid after updates got installed

Summary:
The plasmoid searches for the next batch of updates which might be misleading.

Related: https://bugs.kde.org/show_bug.cgi?id=390143

Test Plan: Installed updates, plasmoid closed.

Reviewers: lukas, jgrulich

Subscribers: plasma-devel

Differential Revision: https://phabricator.kde.org/D10404