Fix media_content_id attribute in Spotify integration (#37853)

Previously, the media_content_id field would contain the track name, which
appears was a regression when the Spotify integration was rewritten in
7e4b9adc. The media_content_id now reverts to containing the Spotify
track 'uri' field, which has the form 'spotify:track:...'

homeassistant/components/spotify/media_player.py

@@ -167,7 +167,7 @@ class SpotifyMediaPlayer(MediaPlayerEntity):
     def media_content_id(self) -> Optional[str]:
         """Return the media URL."""
         item = self._currently_playing.get("item") or {}
-        return item.get("name")
+        return item.get("uri")
 
     @property
     def media_content_type(self) -> Optional[str]: