Skip evicting children that are opening when entering split

- When launching a trampoline activity into split, the trampolining can
  result in the launch of the next activity in the same transition as
  the launch transition (but in an incorrect order), which can result
  in the wrong child being retained (the original activity which will
  end up being closed).  Instead, track all the opening tasks in the
  enter transition (not just the top most), and only evict the other
  tasks but those

Bug: 385674612
Flag: EXEMPT bugfix
Test: Manual, launch trampoline activity into split, quickswitch
      split again with same trampoline activity (repeat multiple times)
Change-Id: Id08cceef94b109cc6bc43add263bbe2ccc5148fc