Bug 921050 - Send email in the background.

Maybe move the sound pref to here (mentioned in other comment). One wrinkle though: right now no sound plays when the background send was for an activity flow, like Contacts -> Email. Once the message reader closes out, the UI switches back to the Contacts, and the send happens in the background.

Ideally, we could play the sound in this case, to give the user a confirmation that the email did send out. That likely needs to be balanced with just a background sync triggering an outbox send, and in that case, we likely do not want the sound to play. May need to work with UX to get the right behavior defined.