iOS Song cleanup

Removed the pbulic NSUrl constructor for iOS Song in favor of exposing the AsserUrl (it was unused)
Removed the duration return based on the duration of the AVAudioItem as it is null until the song is played
Playing the songs :)
Currently the songs play themselves which is ugly and wasteful. I want to move the player from Song into MediaPlayer, so this property has to be exposed at least internally. There are multiple system level media players and the one we want to use is not something that suits general needs, so exposing the asseturl is a must for us.