Fix pausing a mpris player when another is still playing
Summary
The handling of multiple MPRIS players was wrong in some edge cases. This MR fixes those.
Test Plan
Before:
Make sure one MPRIS is playing. Pause it (directly, or via KDE Connect). Then start another MPRIS player.
KDE Connect's media notification will still show the first player.
After:
Repeat the same actions. This time the notification will switch to the second player (which the user probably wants to control).