Bug 879343 - [B2G][Buri][Unagi][Inbound Calls]User Cannot receive inboun...

...d calls

@etiennesegonzac, due to bug 823958, we will need some modifications in gaia as well. Since bug 823958 guarantees that app will get the event 'callschanged' once the calls array is valid, even if it is empty, we shouldn't let oncall.js handle existing call screen. Instead, I propose to let dialer.js take care of both opening and closing call screen. Would you please help review this? Thank you.