Consolidate leaving split screen to pip mode behavior

Stop always putting another side to the bottom when a splitting task is
entering pip mode. Leave it to onStageHasChildrenChanged to handle, and
prevent expanding non-visible side to the top after dismissed the split
screen, so flows of leaving split to pip like swipe-to-home or launching
another fullscreen app won't expand another side to top unexpectedly.

When swipe-to-home from a split pair, it dismisses split screen.
However, it also performs fling divider to dismiss animation if one side
of the split is empty due to entering pip. Which might be conflicting
with each other and leaves the split layout modified with the fling
animation but not preperly reset divide position. Update to make sure it
always resets divide position and cancel running fling animation after
split screen dismissed.

Fix: 227727673
Test: atest WMShellUnitTests
Test: verify entering pip from split behavior with back-key, home-key
      and launching another fullscreen app.
Video: http://recall/-/fLARJNt42LVxc3tt86SneW/gvATxkQhdJ0APek6RwRHel
       http://recall/-/fLARJNt42LVxc3tt86SneW/QfHt9H0XtDNDSP3h1AA8K
       http://recall/-/fLARJNt42LVxc3tt86SneW/QnbdeWo2007s8ZuMf1Syn

Change-Id: I2c53ed21e4766d06180e670ee21534cd622db081