Add a polkit rule to allow background metadata/appstream updates

for inactive users.

- Log in any two users.
- Terminate mintupdate but remain in that session.
- Delete ~/.cache/mintinstall/pkginfo.json
- Run 'sleep 10 && mintupdate'
- Switch to the other session within 10 seconds (ctrl-alt-f7/f8?)
- Switch back after 20 or so seconds, observe authentication
  prompt waiting to download appstream info during flatpak updates
  check.

Ref:
linuxmint/cinnamon#12847
linuxmint/cinnamon#12728

In my testing, I was unable to reproduce some of the side-effects
reported (having to hard-reset, etc...) - I was able to cancel or
complete the authentication dialog if I switched back to the test
session soon enough, but this should hopefully avoid those prompts
appearing altogether.

Author: mtwebster

etc/polkit-1/rules.d/mintcommon-unattended-meta-update.rules

@@ -0,0 +1,8 @@
+polkit.addRule(function(action, subject) {
+    if (action.id == "org.freedesktop.Flatpak.metadata-update" ||
+         action.id == "org.freedesktop.Flatpak.appstream-update") {
+            return polkit.Result.YES;
+    }
+
+    return polkit.Result.NOT_HANDLED;
+});

test

@@ -3,3 +3,4 @@
 sudo rm -rf /usr/lib/linuxmint/common
 sudo rm -rf /usr/lib/python3/dist-packages/mintcommon
 sudo cp -R usr /
+sudo cp -R etc /