Bug970187, CDMA-3way calls

https://bugzilla.mozilla.org/show_bug.cgi?id=970187
Can you please turn this check into a function like cdmaCallWaiting() does? It would be nicer to have something like this a few lines above where we're already checking for CDMA call waiting:
```
if (cdmaCallWaiting()) {
  handleCallWaiting(telephony.calls[0]);
} else if (cdma3WayCall()) {
  CallScreen.hideAddCallButton = false;
}
```
And then:

```
function cdma3WayCall() {
  return isCdmaNetwork() &&
         ((telephony.calls.length >= 2) ||
          (telephony.conferenceGroup.calls.length > 0) ||
          (activeCall() == null));
}
```