kiji-schema: SCHEMA-883. Change Access Modifiers EntityId Implementations

issue: https://jira.kiji.org/browse/SCHEMA-883

Changed access modifiers of EntityId Implementations

Changed access of FormattedEntityId, HashPrefixedEntityId, HashEntityId
from package-protected to public to allow Kryo serialization of EntityId
from outside of Kiji-Schema

review: https://review.kiji.org/r/2075
commit: https://github.com/kijiproject/kiji-schema/commit/c8edc4294c31dcfb98f8cd2c277ee995aafa5d42