Animation cleanup

This adds missing nullability and IntDef annotations, fixes invalid
javadoc references and typos, resolves VisibleForTesting visibility,
marks fields final where applicable, and restricts visibility to avoid
exposing internal at a higher than intended visibility level.

Flag: EXEMPT cleanup
Bug: 281029564
Test: presubmit
Change-Id: Ib35319303e4a354599a345525424e3043379bae5