Apply a full shadow to HUNs when the shade is closed

When a HUNs is displayed in the shade and it is overlapping with the
scrolling stack, its shadow is interpolated by the overlapping amount,
to achive a nice transition as the overlap amount reaches 1.0f.

This calculation broke when they shade is closed, which resulted in a
little or no shadow at all. This CL fixes it by skipping the overlap
calculation when the shade is closed, so StackScrollAlgorithm can
explicitly set the full shadow.

Fixes: 440572084
Test: receive a HUN when the shade is closed -> observe the shadow
Flag: com.android.systemui.notification_fix_hun_shadows
Change-Id: Ia8c42e498ab70ef94e0804690ecde3ed47fa0a89