InCallUI: add decline and answer  incoming call action on notifications

Add decline and answer call action on notification if an call is incoming. Especially
usefull for call in background feature to have the ability to directly
decline or answer a incoming call. But as well usefull to decline/answer a second call
during one is incomming.

Change-Id: I92d8f5a2de74f9877a61c4da4c3eaf2e25230895