Fix launcher crash by set divider hidden

We will hide divider when isLikelyToStartNewTask become true, but
this call sometime earlier than onRecentsAnimationStart then cause
crash because mRecentsAnimationTargets is still null.

Fix this by checking mRecentsAnimationTargets before set divider
visibility. And also add new condition to hide divider to ensure
it hidden if such case happened.

Fix: 265238266
Test: manaul
Test: pass existing tests
Change-Id: I80b1294e69a52e7ac5255cd8e55e7c5e6a3dcbcb