SetupWizard: Don't reprompt setup if account already exists.

  If a user goes through the gms flow, logging in, disabling
  backup, getting kicked over to restore, and deciding to
  set the device up as new, the existing logic would reprompt
  since the flow causes the load action to happen again.

  Now check for account existance, and skip the setup if true.

Change-Id: I3502294d54fcb4e202dca37ff7c6165da4401808
TICKET: CYNGNOS-2459