Keep z-order of tasks when merged into recents transition

This keeps the z-order of incoming tasks during merge the same
as WM. This way, translucent tasks will still draw above their
siblings.

Bug: 289131212
Test: use 3p launcher, open translucent task, switch to opaque
      task, enter recents and then hot-launch the translucent
      task.
Change-Id: I75dc9972b0651f93d0f5c505b90493761cae0843