Fix NSSL bottom margin with 3-button nav and no split shade

It looks like some of the margin calculation logic in
NotificationsQSContainerController.calculateBottomSpacing got mangled
when we added split shade, so make sure there's enough space to swipe
the whole shade up even with 3-button nav and no split shade.

Bug: 222594381
Test: manual, on Pixel 5a
Change-Id: Id83298f7412d087b4b8a6211587b4ec32710315b