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

@shane-tomlinson feedback appreciated!
Setting the currentAccount from this.uid seems like it would eliminate the need for [a lot](https://github.com/mozilla/fxa-content-server/pull/1987/files#diff-0dc8124c36f55037d013efdc72d8eaa8R108) of [changes](https://github.com/mozilla/fxa-content-server/pull/1987/files#diff-1e7023ec1cef9d94ba560d6b63545743R43).