Ensure getParentSurfaceControl and getSurfaceControl return valid or null SC

Many places (like SurfaceAnimator#reset) expect that these SC will be valid or
null, but various implementors of Animatable were not fulfilling this contract.
Fulfill the contract and update the java-doc. Since everyone follows function
documentation this will never happen again.

Test: Existing tests pass. go/wm-smoke
Bug: 129949355
Change-Id: I5acb6b3209158c4c5b258a06c5d60d9e06bb6db1