Bug 1037054 - FxOS Loop client app bits for bug 1016277

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1037054 please.
> Should we destroy this audio context when leaving the competition?

Not sure to be honest. A member of the audio team granted feedback about this helper and didn't mention it. I guess we should.

> If not, why are we creating it everytime we compete? Should we reuse it?

The helper added here was also added to the callscreen app. There was a need in that app that needed the helper to create a new AC when competing to implement a 'force win' mechanism. I realize we don't need that here so we could create the AC just once. I'll change it.