Adds gap after high-priority notifications.

This change adds a gap between notifications of IMPORTANCE_DEFAULT or
higher and all other notificaitons.  The position of the gap is
recalculated from NotificationPanelView.updateNotificationViews() and
the various components which handle the rounded corners of the
notification shade (NotificationRoundnessManager, the shelf, and the
drawBackground() method in NotificationStackScrollLayout) are updated to
handle rounding the corners on either side of the gap.

Change-Id: Ib8d875d4b82184c9c4d363669606261030a467c8
Bug: 116622974
Test: atest SystemUITests