SysUI: Don't try to put non-resizeable task in split-screen

WM would just ignore the request anyways.
Also, move secondary split task forward when entering split so random
non-resizeable task doesn't end up on-top.

Test: Repro. steps in bug.
Fixes: 154362321
Change-Id: I552f4ebbb132eb1944e42e53563f75a109b25815