Add album art to mpris network packets.

Summary: Sends the album art url, so that the android app can display the album art (by fetching it from the internet). Transferring local album art is not supported yet, but can be added in a future diff.

Test Plan: Works for me in players with/without album art and with/without local file album art.

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Differential Revision: https://phabricator.kde.org/D9563