Improve algorithm for deciding which dim layers to animate.

We always want to animate non full screen dim layers, even if there
are full screen dimlayers registered. If there are full screen dim
layers we prefer the one that is currently dimming, otherwise any of
the full screen dim layers (since they are shared).

Bug: 24668543
Change-Id: Icfd5cca83fc371c641ceb2e7a7b62178535984b7