Refactor TaskStackContainers#assignStackOrdering()

- Instead of nested loop over state and TaskStacks, bucket the TaskStacks
  based on its state first, then iterate over the buckets later.
- Stabilize the z-order of the split screen divider anchor and the
  animation layer.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: Id37ecdd6f310ee5a9f7b8d93f65bea564ac95752