Skip launcher animation when top window is already animating.

Currently we invoke the launcher back callback even
when the top window leash is not created (which could happen when it
already has an animation in progress, such as activity launch
animation).

Bug:231636972
Test: Swipe back during activity launch animation. m

Change-Id: Icf9a39bf68cb862defde3c2cfa5960eada41d277