More SurfaceAnimator lifetime fixes.

1. The last fix to Dimmer was insufficient, the mDimLayer in the context
   of DimState() is not the mDimLayer that we return from DimAnimatable#getSurfaceControl.
   Here we null the correct layer.
2. This bug is hard to fix without repro so we add some isValid checks preventing further crashes
   at the cost of a few cycles and a code wart.

Test: Existing tests pass
Bug: 130315011
Bug: 130364451
Change-Id: I3d4d3e0e7110f943e5a2614e07a1f4ae85c2b3da