Fix support for split-screen w/ non-resizable home

First, this treats non-resizable minimized the same as
resizable minimized except with a status-bar height primary.
This differs from before in that the dividerbar remains
visible/usable -- this makes it more obvious to the user that
they are in split-screen mode.

Second, this actually places the home stack into the split
secondary root and overrides its windowing-mode to fullscreen.
This is needed because otherwise it can't properly interleave
with the other secondary tasks -- which would cause backing-out
of the secondary task to return to recents instead of home.

Both of these combined also allows us to clean up some
special-case code.

Bug: 159247878
Test: Use split-screen with non-resizable 3p home.
Change-Id: Idc2050703d972a4b2fa8f74f5827bcc126dce832