Patch: fix incorrect listenbrainz submissions

The function `emms-browser-add-tracks-and-play' switches tracks before
invoking `emms-stop'. This causes the hooks in
`emms-player-stopped-hook' to run after the current track has changed.
This means that, when `emms-listenbrainz-scrobbler-stop-hook' is
called, it submits the track we switched to, not the track that's
playing.

This patch fixes that by storing the relevant track instead of
depending on the currently playing track.