Get account and Google Drive connection summary

Call once at the start of a session to confirm which FindMe account, plan and Google Drive connection are in play. accounts_match is false when the FindMe email and the connected Drive Google email differ (Drive imports then only see folders that Drive account can access), and null when Drive is not connected. hint is a ready-to-show sentence, or null when nothing needs attention. drive.tier_allows_drive says whether the plan includes Drive imports.