In order to sync, a background worker needs to have access to the current account.
We could serialize the account information as part of the Data object that the worker
will operate over, but that's potentially problematic, as we'll inevitably risk our
serialized account information will become stale by the time worker runs.

This means that it's much better if we can just create an instance of the current account
inside the worker. For that, it needs to have access to the account storage object mechanism
that's used within the AccountManager. Rather than dealing with making SyncManager
configurable with different types of account storage implementations, let's assume
we're going to use a single one, defined in the services-fxa module. This makes
our life easier for now, and is easy enough to re-visit in the future if we need to.