Change stopped to STATE_IDLE instead of STATE_OFF

[apetrycki]

Description:

Putting the player in the off state doesn't allow you to change playlists from the media_player card, so you cannot turn mopidy on from a clean state using the card. Changed the "stopped" state to STATE_IDLE instead so functions are available from the card. Play also now loads the first playlist if no songs are on the track list.

Checklist:

• The code change is tested and works locally. Tested with mopidy running locally with and without songs in the track list.