Fix missing space between umo and shelf

StackSizeCalculator needs to account for possible gap
before the shelf, in case the first element in the shelf
starts a new section.

Fixes: 227107113
Test: manual
	=> see gap before umo and shelf
        => no gap between shelf and previous notif if
		previous notif and first notif in shelf
		belong to same section
Test: NotificationStackSizeCalculatorTest
Change-Id: I8724f81327610a49a95b2aff69846bb06119c054