Reset windowing mode for split when starting existing tasks

* Sets windowing mode to undefined so task can assume windowing type
of parent.

Test: Tested w/ existing tasks in desktop and non-desktop
Flag: com.android.wm.shell.enable_flexible_two_app_split
Fixes: 381954590
Change-Id: Ic91785702a599d8f3c808e885d9cf43c0469ff5a