Keep FSI HUNs visible in the shade regardless the scroll position

HUNs with an FSI will remain pinned, regardless whether the shade is
open, or if we've called ENR#markHeadsUpSeen() on them.
Pinned HUNs are always clamped to the minHeadsUpInset, meaning that they
can never be overscrolled offscreen, but their z-translation was never
adjusted, after they have been marked as seen. This was causing a visual
bug, where we were scrolling stack's content collided with the pinned HUN.

This CL changes fixes the z translation by making the FSI HUNs return
true for ENR#mustStayOnScreen(), thus forcing the StackScrollAlgorithm
to keep it visible to the user.

Fixes: 385041854
Flag: com.android.systemui.notifications_pinned_hun_in_shade
Test: ensure that FSI HUNs are displayed correctly in the shade
	- open the shade
	- scroll up the stack
	- receive a HUN with FSI
Test: ensure that regular HUNs are displayed correctly in the shade
        - open the shade
	- scroll up the stack
	- receive a HUN (without FSI)
Change-Id: I32b3ba20d2b42c8c984575b702b5c54db27912af