AccountService: Implements default account failsafe

Implements one of the fixes outlined in issue #2637.

/cc @sendhil + @astralbodies + @koke 

@jleandroperez - I'm wondering but not too sure about this: since previous app versions store the default account under the key `AccountDefaultDotcom`, and the new version uses `AccountDefaultDotcomUUID`... won't that effectively log the user out when updating the app?