Bug 1016277 - If two different apps try to use the 'telephony' channel at the same time both apps can play audio

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1016277 please.
oh just remembered, we need to set the channel when creating the context -> `new AudioContext('telephony')`.

Cf the description in https://bugzilla.mozilla.org/show_bug.cgi?id=997701