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

@shane-tomlinson feedback appreciated!
@zaach - that sounds like a good plan. user is passed in to both `signIn` and `signUp` in fxa-client. After both complete, they both call `_updateAccount`, which calls `user.setCurrentAccount`. A better route forward, as you suggest, may be to call `user.setCurrentAccount` from the views. fxa-client->`signIn` and `signUp` could return the updated session data that is used to set the current account.

I still feel like there is a lot of business logic that should be in neither fxa-client nor the views, some middle abstraction that lets views take care of view things, and fxa-client with communicating with the fxa-js-client.