Document behavior of object references in Animator classes

Animators that are created with Object values may have undefined
behavior when those objects are changed outside of the animator itself.
For example, an animator created with a start Rect and end Rect
will animate the bounds between those two objects. But if the caller
changes either the start or end rect before or during the animation,
that will affect the values used on every frame of the animation.

Issue #28304520 Strange interpolation of property values holder values

Change-Id: Id6ac19c8369ae34450e6b53d68f5e492b27c577e