Update DimAnimator layer when its window changes.

Prior to this fix once dimming had been turned on it stayed at the
same layer and associated with the same window until it was turned
off. Now the DimAnimator layer is updated if either the window layer
changes or the dimming window changes.

Fixes bug 6467865.

Change-Id: I3e1765b92b51be26e3718c8a87e2583041a36af9