Only reparent controlling tasks when entering split

Only move split controlling tasks to split secondary root when entering
split. So that it could align with launch root rule while in split.

Fix: 187892355
Test: enter split screen while having assist type task in recent,
observed it won't reparent assist type task to split-secondary-root.

Change-Id: I648dc74e2e44d62a0e40d50b29b08dd23b85ed26