Don't rely on stale DP.isLeftRightSplit for app pair landscape launch

* If phone auto-rotate is on but launcher rotation is off, if you
launch app pair holding phone horizontally the apps will launch in
landscape but DeviceProfile will report portrait.
* Now we see if all the leaf tasks have their top bounds be 0 to
determine leftRight split-ness
* Animation is still wonky, but acceptable for this edge case for now

Fixes: 356763330
Test: No crash, app pairs launch fine in normal portrait and landscape
Flag: EXEMPT bugfix
Change-Id: I0a067b50751b919bda5446c3e36055e58222bcab