touch-extrausers: also add pam_extrausers into "account" PAM stack

account PAM stack is responsible for "non-authentication based account management". Lomiri calls this PAM stack after the auth stack to check if the user is allowed to login/unlock.

Without pam_extrausers in this stack, the main pam_unix will have to handle this task for phablet user, despite the main /etc/passwd not containing this user. Apparently pam_unix has been more lenient with respect to missing user in the past. However, this is no longer the case and pam_unix now returns failure for non-existent user.

Thus, let pam_extrausers also handle account PAM task for phablet user by adding it to the account stack.