Fix for iOS Song's Play/Stop

In XNA, calling MediaPlayer.Play() should start a song from it's beginning. AVAudioPlayer does not have the same behavior, and needs it's CurrentTime property set to 0 to emulate this behavior.

Replication steps:
Play a song using MediaPlayer.Play().
Play a second song using MediaPlayer.Play().
Call MediaPlayer.Play() again on the first song played
Bug: First song starts at where it left off, rather than the start.