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

@shane-tomlinson feedback appreciated!
> Nevermind what I said in the meeting, I think this would work for all the broker methods.
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.