Fix split divider flash if swipe again quickly

This bug caused by divider will be set to show when animation
cancelled, because split screen will show divider itself when
both split visible, this call might be unnecessary. Fix it by
removing these lines.

Add surface valid checking when running divider animation to
avoid crash caused if surface was released during animation.

Fix: 242823773
Bug: 246371786
Test: manual
Test: pass existing tests
Change-Id: I10aaf86e4164327b1c6c2f323c62b246c600228a