polkit: fix CVE-2019-6133

Jann Horn of Google found that Polkit doesn't properly check
if a process is already authenticated, which can lead to an
authentication reuse by a different user[0]. See also [1]

Closes #55391

[0]: https://bugs.chromium.org/p/project-zero/issues/detail?id=1692
[1]: https://gitlab.freedesktop.org/polkit/polkit/issues/75