This PR corrects `GetSeed` to be easier to use and less bug prone. This is being done in preparation to the `os_crypt_sync` migration. An interface change was the addition of `IsEncryptionAvailable()`, which will be useful when this type is given an encryptor. This PR also removes a lot of the os crypt setup and tearr down for tests, to make it easier to have them removed once they are not needed anymore.