Bug 1069793 - [Loop] While GSM call is established, Loop call can't be established

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1069793 please.
If there is no async. code in compete (and it seems there isn't), this patch would be just adding ` AudioCompetingHelper.compete()` before asking for gUM, keeping https://github.com/mozilla-b2g/firefoxos-loop-client/pull/131/files#diff-2fbba884ef4a966212c06b83fa8ebd85R48.