Use same calculation for portrait as fake landscape for layout of split thumbnails

* Partially revert ag/20330754, not entirely sure why we switched from
using dp.isLanscape to splitInfo.appsStackedVertically, since the two
represent different states (specifically for fake landscape)

Fixes: 280783620
Flag: None
Test: Tested on foldable, small + large screen.
Fake and real landscape on small screen.
Rotated while in overview.
Slight sliver of wallpaper shows on large screen, but that
issue has been there.

Change-Id: I13d834a31417751dd3b757344ec7b4d9a174090e