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

@shane-tomlinson feedback appreciated!
> Nevermind that nevermind– I remember what stopped me from doing this in the first place. The desktop broker sends `accountData` to the desktop in a few cases, some of which overlap with the web channel broker. It seems less confusing to me to keep these methods consistent and use the more general case.

Ahh, I see, the fx-desktop broker uses `accountData`. Yer right. Carry on.