Update SurfaceView.getSurfaceControl documentation

Updates the javadoc to clarify that the returned SurfaceControl is only
valid between `surfaceCreated` and `surfaceDestroyed` callbacks, and
will be null otherwise.

Bug: 442013170
Flag: DOCS_ONLY
Test: presubmit
Change-Id: Ieed0f82b3335deda67a6717ced70143fac99f3c7