Use alpha instead of show/hide when creating animation leashes.

Relative layers inherit visibility flag from the layer they are relative
to. This caused dim layers to disappear when the dim layer was transferred
from one window to another that was starting an animation. The animation
leash was set to hidden so the dim layer was also hidden.

Instead set alpha to 0 and 1 when trying to show and hide the animation
leash. This will not update the dim layer's visibility since only the
show/hide flag is inherited.

Test: Steps from bug
Fixes: 138337187
Change-Id: Id7d82335889a42020ad0adccf9bd5c5d987cc813