account: Make admin account usable in case of errors

In gis-account-page-local.c, if parental controls are enabled we create an administrator account which will be used by the parent, and we create it before the child's account so that the system will not be left without any admin accounts in case g-i-s crashes or otherwise doesn't finish. However this isn't especially helpful since the account is not set to be passwordless (ACT_USER_PASSWORD_MODE_NONE) so cannot easily be used if the user doesn't make it to the page where they are asked to set a password on it. So, set the admin account to ACT_USER_PASSWORD_MODE_SET_AT_LOGIN temporarily. When a password is set on the account by the password page, accountsservice will set it to ACT_USER_PASSWORD_MODE_REGULAR.

This seems like a slightly better policy than making the account passwordless, since we shouldn't encourage passwordless admin accounts.