Adding new CHANGING transition to LayoutTransition.

LayoutTransition used to depend on child views being added/removed or
shown/hidden in the transition container. These evens would trigger animations
to fade the child view as well as those to animate the side-affected changes
to sibling views. This CL enables a new feature in LayoutTransition that
enables animating any changes to the layout of the children in the container
whenever a layout occurs. For example, you can change the LayoutParams of a
child view and call requestLayout() to automatically animate those changes.

This capability is not enabled by default. To enable, call the new
LayoutTransition.enableTransitionType(LayoutTransition.CHANGING) method.

Change-Id: I4d07a3b36245353b2151f0dca4f75080ab6a4592