1256459 - Refactor FxAccounts to better reflect client states

Status: NEW

(Firefox :: Firefox Accounts, defect, P3)

Description

[Mark Hammond [:markh] [:mhammond]]

The state management in FxAccounts is ad-hoc and thus error prone. This leads us to bugs like bug 1254810 which reflects confusion about what data is valid and what data should be discarded under some error situations. We should refactor this to reflect https://wiki.mozilla.org/User_Services/Sync/FxA_Client_States.

Off the top of my head, we could consider:
* Change getSignedInUser() to only return (say) email and uid fields. This then remains the "is a user logged in?" entry-point.
* Add a new entry-point that is used to get named field(s) for the signed in user (ie, the caller must specify exactly which fields they care about).
* An internal state machine and mapping of field names to states.
* Thus, callers don't need to know about these states - FxAccounts will automatically transition through the relevant states to obtain the requested data.