InCallUI: add decline and answer  incoming call action on notifications (1/2)

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.

PS:
- found a better way for the answer button

Change-Id: I92d8f5a2de74f9877a61c4da4c3eaf2e25230895