kiji-scoring: SCORE-139. Add static constants for fresh and stale to KijiFreshnessPolicy.

issue: https://jira.kiji.org/browse/SCORE-139
review: https://review.kiji.org/r/1423
commit: https://github.com/kijiproject/kiji-scoring/commit/273be929903801baa2a2e2e3fde691db413f900c