Fix accident evict if drag same app to same position.

We evict all children on current root when start intent to split.
But if drag same app, it will not launch any new task so all task
under root are evicted.

Fix this by doing evict job on startAnimation because we could
get all transition change here. We only evict tasks under split
root if there have new task launched.

Fix: 279873859
Bug: 279063348
Test: manual
Test: pass existing tests
Change-Id: I4793133f060faf628ba85feb5123a2aaf5e0f7fb