Skip onTaskMovedToFront if the bubble selection is animating

When a bubble gets dismissed while the stack is expanded, we should
select the next available bubble. Instead, we are collapsing.
This is because we get an onTaskMovedToFront event for the app
behind bubbles moving to the front, which causes the stack to
collapse. To fix this I add a check to see if the switch animation
is running, and if so we skip the collapse in onTaskMovedToFront.

Bug: 208648025
Test: manual - have multiple bubbles, expand the stack, visit each
               bubble
             - dismiss each bubble by dragging to dismiss target
             => observe that after dismissing each bubble, the next
                bubble is selected (if available)
Change-Id: If82c004ecd1ec8920e934dbae2107b4c37853cb5