Fixes exception thrown while making non-root task always-on-top

The always-on-top was prevented to be set on a created-by-organizer
root-task via ActivityOptions in commit f3a8b7d6. So, it is now possible
to be set on a non-root task while its parent may not always be
DisplayArea.

Bug: 430687520
Test: wm presubmit
Flag: EXEMPT bugfix
Change-Id: I71804697b301714ab7a7e788b2de6de387e58a32