Consolidate split divider show/hide behavior

Consolidate split divider show/hide behavior by hiding the divider bar
at a single point where the tasks actually start moving. So it won't
need to deal with hiding the divider before the animation targets ready.
Also prevents to hide the divider too early when users were just
unstashing the taskbar.

Fix: 261376202
Test: http://recall/-/fLARJNt42LVxc3tt86SneW/colHl9bXqOzppYV5o2Hmjh
Change-Id: I2b7b37c2b20cc379581b34c0104fa45246c27e8f