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

@shane-tomlinson feedback appreciated!
Please add an additional check here. 

After clearing the pre-filled email address and re-opening FxA, `email2` [was displayed as the address that would be signed in](https://github.com/mozilla/fxa-content-server/issues/1973#issuecomment-67229405), but the RP received a code for `email`. Ick.