We've had separate title & artist for a while, and all clients should be
using those by now.
Also fixes the position change not being emitted when the song changes,
and fixes the values being written after emitting that they changed.
Summary: Current code only supports one player
Test Plan: Run two media sessions. Dbus inspection works fine, UI has some issues with values not updated, but out of scope for this diff
Reviewers: #kde_connect, apol
Reviewed By: #kde_connect, apol
Subscribers: apol, #kde_connect
Tags: #kde_connect
Differential Revision: https://phabricator.kde.org/D12546