Update launch root rule to consider created-by-organizer parent first.

Before this, we use the source root task as a default launch root when
starting an activity. Updated to also consider the first
created-by-organizer parent as a default launch root because the
different side of the split will share the same root task after we
migrated split screen to a single-top root task hierachy.

Bug: 207185041
Test: launch activity with launch adjacent flag will enter split screen
Change-Id: I4db5be181c03141b11c77e8f76b49fa9f24686cb