Model hiding the footer in the NotificationListViewModel

Sometimes, when opening the shade, the quick settings panel would jump
to a new position after the initial animation (see recording in bug).
This happens because with the refactored footer code, we set the footer
visibility to GONE (via StackScrollerDecorView#setVisible) as long as
the shade is closed, which means it was sometimes not being
counted when measuring the height of the NSSL for the animation (in
NPVC#calculatePanelHeightShade). In the old code, we were setting it
to INVISIBLE instead.

This fixes this bug by modeling this "hidden" state that is needed in
the StackScrollAlgorithm in the NotificationListViewModel. I had an
attempt to include it in the VisibilityChange object we've been using
so far for the footer visibility, but that makes things harder to read
since this hidden state is handled differently and without needing to
be animated.

I have an alternative solution for this at ag/26581241 but it's hackier,
so I believe this to be a better approach that preserves the
pre-refactor behavior.

Fix: 329432313
Test: open shade in various ways (different gestures, screen size,
orientation) to check that the bug doesn't happen anymore
Test: NotificationListViewModelTest
Flag: ACONFIG com.android.systemui.notifications_footer_view_refactor STAGING

Change-Id: I8d34e74fc1d3939176132850d1d962ec5c7bccd2