This involves redoing the persistence model, to split general,
account, and folder settings into separate SharedPreference stores.
It also requires some preferences to be moved into UnifiedEmail.
Depends on Ibe2e3f93ec164370535ffc5f5b2409544cc8d36d (UnifiedEmail)
Change-Id: Ie6ec389b5b5d2e7ab1b299d0877811ae716526e2
Our original plan was to disable both authenticators by default, and enable
one of then on boot. However, it turned out existing exchange accounts will
be removed if there's no enabled authenticators for the account type.
So, instead, in this patch we initially enable only the default one, and switch
to the other one on boot if the vendor policy indicates so.
(If a device has a vendor policy apk, it should also have the email app
preloaded, so changing the label at boot time isn't too late.)
Bug: 2382710
