With this CL, we have "All accounts" in the account list and "Inbox" & "Unread"
in the mailbox list. "All" + "Unread" doesn't play very well, so, we should
seriously consider whether it's worth the effort to support it right now.
Change-Id: Ia1c07c89a535a68ffe6f3ae5009b5b6591a9acdb