fix mark1 demo

[JarbasAl]

bus message to sing used by mark1 demo was broken

import of play_mp3 was removed when skill was changed to use audio service

this PR does the bare minimum to make things functional

future work:

• also use audio service on bus message instead of play_mp3
• stop depends on play_mp3, should check what is playing on audio service
• allow selecting song on bus message? demo skill would also need changing

[forslund]

Sorry for missing this PR for so long. I switched to using the audioservice to start the track on messagebus event as well.

The other things looks like good ideas. I'll create an issue with those.