Allow phosh to take over incoming calls when shell is locked

This contains !361 (closed) and extends the DBus interface so phosh can take over incoming calls when the shell is locked (#107)

I'm marking as WIP since the phosh side needs more testing and we want to coordinate the releases. An alternative would be to spit out the DBus changes (which don't break anything) and then just drop the layer surface at a later point (maybe making it a compile time option first)

However let's build testable debs right away.

Phosh side is phosh!780 (closed)