examples: Don't pass multiple policy values to flatpak

At some point, the flatpak command line interface changed so it is not possible to add multiple values to a same key with a single --add-policy call. Doing this now results in a single value added, with the ';' escaped, this breaks Tracker portal access.

It is however possible to call --add-policy multiple times to make a multi-valued key, which provides access to the multiple graphs.

So adapt example to use the working solution.

Based on and quoting from nautilus@9ff35c64