[mobile/wifi] Fix some qml warnings in ConnectionItemDelegate

Summary:
Using anchors on items managed by layouts is undefined behaviour.

This fixes a few warnings and also improves the visual appearance

Test Plan:
On the desktop before:

On the desktop after:

On the phone before:

On the phone after:

Reviewers: Plasma, bshah, jgrulich, mkacej

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22178