Bug 1077768 - Send audio channel infromation to music app for handling u...

...nexpected audio resume.

Mozilla url: https://bugzilla.mozilla.org/show_bug.cgi?id=1077768
Do we have to notify all the audio channel changes to the music app? the `audio-channel-changed` fires not just in the content <-> ringer cases, there are other possibilities which means this will send some redundant information to the music app, if you don't filter the cases.