Reject an incoming call if there is an ongoing Call

[atsakiridis]

Right now is that on second call while 1st is ongoing you get ringing and from then on it messes up: If you decline it closes the 1st call's controls but media is still there. Need to have SDK automatically reject.

[atsakiridis]

Already done some work on it on branch atsakiridis_issue512. Please resume there.