Animate bottom crop boundary to 50% at start.

Previously just sat at the bottom.

Eventually could set this to the "original" bounds, but that's not
simple right now.

Animating in so as to give a clearer hint that that the handles are
movable.

Bug: 179907401
Test: Take screenshot, observe transition.
Change-Id: I757a4433d89beaaf65f34d42229fad19f56acb34