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