KCM: Fix connection list background and padding

Summary: Like D27379 the ListView in the Network Management KCM is incorrectly anchored to its parent ScrollView. This removes the anchoring, removing overlap of the scrollbar. In addition, it corrects the ScrollView's background so it actually displays correctly.

Test Plan:
Before:

{F8101662}

After:

{F8101661}

Reviewers: jgrulich!, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27382