Improve javadoc for rotationAnimation.

Clarified use of rotationAnimation. Did not add a comment
for ROTATION_ANIMATION_CHANGED as that would be inconsistent
with the other twelve <parameter>_CHANGED flags that it
follows in the source code.

Fixes bug 8657715.

Change-Id: I03b5caf3d6a93ca0044f58485c94c7a600e835a8