fix(signin): pass account data to broker instead of using currentUser

@shane-tomlinson feedback appreciated!
I'm confused by this section of code, and why it is here. Can this be done near [where the error enters the module](https://github.com/mozilla/fxa-content-server/pull/1987/files#diff-0dc8124c36f55037d013efdc72d8eaa8R69) to help provide context?