AccountService: Implements default account failsafe

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

If you bump the analytics pod to `0.0.9` you can use this stat here - https://github.com/wordpress-mobile/WordPressCom-Analytics-iOS/commit/29e3fdae58ddb4ec104ff0376c60902ac5960bf3#diff-e080567e1de6ffd66e323565dcd45ebcR52