Update launch root rule to consider candidate task if needed

Bbcd34357, changed the rule for evaluating the root task while
dealing with the candidate task. The source task of the
created-by-organizer parent is considered as the default root task.
The rule changes the behavior in split-screen cases.

This CL considers the candidate task in priority for split-screen as
before.

Bug: 227473044
Test: atest MultiWindowTests
      atest TaskDisplayAreaTests
Change-Id: Ibb75ab0311550d1093c878500e4671f569977753