Set initial position for WindowAnimation and reparent animation leash.

1. Set the initial position for the WindowAnimationSpec to be the stack
position.
2. Reparent the animation leash to mAnimationLayer
3. Updated Surface names for clarity.

Test: Opening apps animate from the correct start location. When apps
are animating, their layers are reparented to the correct animation
layer.

Change-Id: I3e3a1e45f0a0cf9d471dee105abd9bce05d1e91d