Shift Recents task to top when opening Recents.

Unlike usual opening/closing app transition, the Recents task should
above Home task when opening recents, otherwise the home task would
above the recents task so it will seems like the home been add into
recents animation.

Bug: 207297486
Test: enable shell transition, setup 3rd-party launcher as default
home. Entering recents at home page and verify the home task won't
stay on top.

Change-Id: I9bac97cb79febf112ec9ac1bdbe3de205604b01d