Clear child task crop in split so it matches parent crop

When in split, we don't want to give the child task a crop. The
parent has logic how to handle crop and adjust properly when drag resizing.
It's better to just let the child task match the parent task.

Fixes: 175260017
Test: Background appears when drag resizing split
Change-Id: I8f0e3d0773f10889030e0ce4d600dbed2b13ed39