Fix issue where screen rotations would stop animating.

If we went through the update loop multiple times, and had finished
animating the first time, we would blow away that information the
second time and never kill the animation.

Also moved killing the animation back up to the animation step --
this involves destroying surfaces and such, and so really should
be done as part of the surface transaction.  We can also consider
the screen rotation animation object to be owned by the animation,
so it can destroy it when done.

Change-Id: If24356c509c66d046f2ddfd9ad5bfe12504d7716