Since otp_encrypt_api_key only encrypts API keys, it doesn't require
access to the full UserProfile object to work properly. Now the
parameter it accepts is just the API key.
This is preparatory refactoring for removing the api_key field on
UserProfile.
We'll need to implement a version of the simple decoding/decryption
logic used by this library in the mobile code as well, but that should
be simple enough.