Made sure that HUNS with full screen intents stay pinned

When a HUN came in with a full screen intent came in and the shade
was open, the notification was not pinned and therefore got lost
when the shade was closed, potentially leading to missed alarms
or calls.

Bug: 20431566
Change-Id: I0d5da0f9baba8d9f68ba2755c40c5fd9a8471191