Rename confusing animation naming.

The naming was ANIM_UP, ANIM_DOWN, but the animations were from right and
left. Since right and left are flipped for RTL, this CL renames them
with direction agnostic ANIM_ENTER and ANIM_LEAVE.

Change-Id: I8de1603b1baaef3d4ba8fbd593f5e53143698dff