use mirror location metadata from index-v2 in mirror selection

index-v2 fdroidserver#928 (closed) provides the location of the mirror. That lets the client choose which mirror to use based on location without having to query the network. For example, it could be based on the country that the user sets in the localization settings. Or it could be possible to download the geoip database and use that to decide. Or query the IP location using a service on the network.