Update `ExportDataEnvelope` `State` types to align with the types defined by the server. Update related logic to be more readable. Also update some test cases to default to `none` instead of `expired`, now that `none` is the default state for users that have never requested their data before.

Update the ExportDataEnvelope `state` enum to rely on an underlying `stateString`. This allows a default `unknown` value to be used if the state from the server is missing or doesn't match a value in the enum.