Prefer selecting the playing player in the MPRIS activity

Summary: Previously, the MPRIS activity just showed the first player in the list when you opened the activity. Now it shows a playing player, when possible.

Test Plan: Tested with zero, one and multiple players, and with both playing and not playing players.

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Differential Revision: https://phabricator.kde.org/D10821