[TIMOB-8670] iOS: When playing sound over existing audio (user playing ipod,pandora,etc) it should lower volume of audio until sound if complete then raise volume back to same level.

*Expected Behavior:* If a user is playing music (ipod,pandora,etc) and I play a sound in my code using Ti.Media.CreateSound I would expect the music to fade to a lower volume, play my sound, then music volume fade back in. *Actual Behavior:* Unfortunately what happens (assuming you are using AMBIENT as audioSessionMode) is they are played together which sounds pretty bad. *Example* To see an example of this first hand. Please do the following: 1. Download RunKeeper from the AppStore 2. Make sure music is playing from ipod or pandora or another 3rd party app. 3. Click 'Start Activity' button in RunKeeper. You can see that RunKeeper will fade out the music, play there sound clip, then fade back in the music.