solenv/flatpak-manifest.in: incorporate upstream sandboxing improvements

...done by Bartłomiej Piotrowski <b@bpiotrowski.pl>,
<https://github.com/flathub/flathub/pull/127/commits/83a660a7c2876cfa53b86a3a5164908d4490a5f7>
"Remove access to the system and session buses" and
<https://github.com/flathub/flathub/pull/127/commits/e8481f172e03542e5be0bde537f62999db7277fe>
"Own correct D-Bus name on the session bus".

Change-Id: I3d58833e0cee64b1c936f76df90a892bc6ce70d4

solenv/flatpak-manifest.in

@@ -403,9 +403,8 @@
         "--socket=x11",
         "--socket=wayland",
         "--socket=pulseaudio",
-        "--socket=system-bus",
-        "--socket=session-bus",
         "--filesystem=host",
-        "--env=LIBO_FLATPAK=1"
+        "--env=LIBO_FLATPAK=1",
+        "--own-name=org.libreoffice.LibreOfficeIpc0"
     ]
 }