Fix HUN pinning in split shade

When notifications are scrolled in split shade,
for incoming HUNs that get pinned, fix:
1) Wrong inset
2) Flicker when scrollY goes back to 0

Fixes: 288553563
Test: in split shade, scroll notifications up, send delayed HUN
      => collapsed HUN pinned to correct inset

      scroll notifications down so that scrollY=0
      => HUN expands in place with no flickering

      do the above in non-split shade
      => no regressions

Flag: NONE
Change-Id: Ibf843e8c195aa7140c00dde0dacdf00eb3654af9