appstream: fix screenshot URL and size

Summary: Use the canonical location for the screenshot, and fix width/height to match its actual size.

Test Plan: `appstreamcli validate src/data/org.kde.kaddressbook.appdata.xml`

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: dvratil, pino, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28648