Refactoring UserIdentity logics

[sunnywu]

0. Read changes here first
1. Removed the UserIdentity's refreshedAt field as that's redundant and currently set to the same value as the AdvertisingTokenInput/RefreshTokenInput#createdAt timestamp.
2. Renamed some token variable names to adTokenInput and refreshTokenInput to make it clearer
3. Removed privacyBits and establishedAt from UserIdentity/HashedDiidentity/RawUidIdentity but only kept inside FirstLevelHashIdentity - these fields are only relevant when a first level hash is generated for token generation and then passed along into ad token/refresh token generation logic.
4. As a result, privacyBits and establishedAt are generated from either a. in a brand new token generation call (by default should be 1 and the timestamp at the time of the call) b. or, during token refresh logic, these fields will be inherited from previous refresh token.
5. RefreshTokenInput, AdvertisingTokenInput and IdentityRequest will now require to store privacyBits and establishedAt itself as a compromise.
6. generateIdentity will now take in a privacyBits param.
7. Updated a lot of codes as a result (more refinement required such as not hardcoding the "1, Instant.now()" in a lot of places)
8. Created verifyFirstLevelHashIdentityAndEstablishedAt method for verifying FirstLevelHashIdentity and added more checks for FirstLevel and check the correctness of raw Uid/First level hash in various unit tests
9. Added PrivacyBitsTest/IdentityResponseTest/RawUidResponseTest classes and added some extra check for firstLevelHash in existing tests
10. Replace privacy bits type to PrivacyBits class instance

[mcollins-ttd]

Maybe this should be PrivacyBits instead?

    public static final PrivacyBits DEFAULT = PrivacyBits.fromInt(1);

[sunnywu]

removed all uses of int PrivacyBits and use PrivacyBits class instead. Added unit tests to test the class