Fix split screen divider corners flicker

Divider bar might be hidden before swipe-up gesture started when users
touching task bar region. This makes sure to update visibility of
divider bar only after gesture started.

Fix: 219995626
Test: long press task bar region to stash/unstash task bar, divider bar
      won't be hidden as long as it didn't trigger swipe up gesture.
Change-Id: Iacf690c84a3ad4b5e4fc9b066e9a1ecb8a8aa7d1