IMS-VT: Show upgrade to video notification.

Upgrade to video notification is not shown
when UE's screen is on and InCallUI is not
visible, e.g. pushed to backgroud by pressing
the Home button.

CL: 121482384
BUG=28093622
Change-Id: Ia141886b7a085164b3294a682f14b60b7a2a83b4