The backend gets to "prepare" the configuration before Configure is
called, in order to validate the values and insert defaults. We don't
want to store this value in the "config state", because it will often
not match the raw config after it is prepared, forcing unecessary
backend migrations during init.

Since PrepareConfig is always called before Configure, we can store the
config value directly, and assume that it will be prepared in the same
manner each time.