[TIZEN] Add media class property in manifest for audio streams

This change extends the manifest with 'xwalk_media_app_class' property that can be used to tag audio streams in pulseaudio and Murphy, so that the Murphy resource policy daemon can dictate when the audio will be paused and resumed in response to other audio playing.

BUG=XWALK-2788
what's the type here? smart pointer? looks like it's better to be explicit here