Loop in reverse when updating task stack.

Changes are sorted from top to bottom. So when we are updating changes, traverse them in a reverse order. This
ensures that when we are tracking the z-order of tasks, the top task is
added last.

Test: Check manually that the changes are comitted in the reverse order
Bug: 414808458
Flag: com.android.window.flags.enable_windowing_task_stack_order_bugfix
Change-Id: I48a798b9d07131a35732f0c0ffe90df432edf4c0