The serializer class lives in OAuthTokenCache.java so the extension
framework doesn't acquire a dep on the cache serialization code, which
currently lives in the server package.
Proto3 doesn't have optional fields, so we can't distinguish between
empty and null for the provider_id field (at least without adding a
separate has_provider_id field). Explicitly treat these the same in
OAuthToken itself, and clarify that other fields are not nullable.
Change-Id: I37bd47ac0c8491e5212fcec4885e063ae50d714d
e6930d73b0