Use list instead of array when filtering non-app leashes

* Non-apps leashes can contain non-divider targets, which
was creating null elements in the array when an index didn't
get assigned.
* With a list we don't have to worry about empty index gaps
* Also remove the animation for the divider for certain
gestures because the surface isn't always valid for the
full duration of the animation. We probably would need to
synchronize with rest of recents animation

Fixes: 212218930
Test: No longer crashes when swipe up, hold, then swipe down
Change-Id: Ia1fc4d66e73f21b55fdbfe59342af025e2a525d9