/usr/share/accountsservice/interfaces/com.canonical.unity.AccountsService.xml is in unity8-private 8.12+16.04.20160401-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/accountsservice/interfaces/../../dbus-1/interfaces/com.canonical.unity.AccountsService.xml