Comment 6 for bug 1419645

We would need input from design, but ofono sends a signal over dbus when a call ends http://pastebin.ubuntu.com/14600960/
We could have unity8 listen for this and restore focus to the n-1 app in the application manager stack. Obviously there are a lot of edge cases here, but this wouldn't be _too_ hard in theory and any 3rd party dialer app (that uses ofono) would behave the same. An API would probably be required for voip apps though.