Invisible views can't start animations

The invalidation logic was broken for invisible views, causing
a problem when an animation was set to start on an invisible view
(to fade it in), but the view would not propagate the invalidation
event that would trigger the start of the animation rendering.
Fix: reset the DRAWN flag when a view becomes invisible to allow
the invalidation logic to work as intended.

Change-Id: Ibf49c6e8b39506a92a6e5f45cb8a12626ae4f931