Default hide animation leash of opening tasks.

RecentsTransitionHandler doesn't know the initial tasks position when
tasks appeared, only RecentsAnimationRunner know. So there shouldn't
show the animation leash immediately, otherwise the opening tasks could
shows up in the final position, before RecentsAnimationRunner start
the opening animation.

Bug: 277704255
Test: manual, entering recents, launch another task, verify no flicker
during the opening animation.

Change-Id: I1c384b825348d8a67d49c4cec89a61a9a6f9687f