AccountService: Implements default account failsafe

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

Not sure about that `ascending:NO`. If there's a WordPress.com account that has no jetpack blogs it's likely it was the default account.

Actually, I don't think I'd use sort descriptors at all.

Fetch all wp.com accounts. If there's one with `blogs` and no `jetpackBlogs`, that's the one

Otherwise there's no easy way to know what happened and I wouldn't touch the accounts. You risk breaking things even more:
- You can have a .com account without blogs for just reader
- If the migration made the jetpack account the default, a syncBlogs might have triggered. Then the jetpack account could have more blogs than the previous default account

Other ideas:

- If we have a push token, ask wordpress.com to which user it belongs to
- An account also has reader sites/topics associated, maybe there's info there to guess which account is the default
- Parse the app logs for hints
- Ask the user
- Declare defeat and sign out