fix(signin): cache credentials for desktop sign-ins, otherwise only cach...

...e email

fixes #1621

@ckarlof r?
When we bolted on FxA to Sync, we really didn't know what we were doing, but had some idea that FxA would want to know the relier/service and the context that login was happening in, hence the two variables. There is currently one way to log in to Sync with FxA on Desktop: `fx_desktop_v1`, but that could change.