Swap z-order of tasks for default wallpaper-open animation

the defaulthandler's wallpaper-open animation reverses the
normal closing/opening task dynamic: normally we animate OPEN
as the opening-task appearing over closing; however, for the
wallpaper-open case (which presumably assumes that the "opening"
task is a 3p launcher), we actually animate the closing task
closing over the opening-task. This requires the z-order to be
swapped.

Bug: 279680678
Test: use 3p launcher and 3-button nav. open an app. press "home"
      button.
Change-Id: I37362c1ade5ee786ed93b0544d37b6b38a549f04