change default app update behavior use latest versionCode

By default, the XML generated from metadata that does not set Current Version Code defaults to not having automatic updates (i.e. <marketvercode>0</marketvercode>). I propose to switch that behavior, and have the generated always include the latest version in the repo as the "current version" unless Current Version Code has been set in the metadata. Even after all the work I've done on fdroid, I didn't know about this option until this week. I assumed that the repo was behaving like I described.

I think that's the natural assumption: I add a new APK, it becomes the latest version. So it would be nice if the default was the same. As far as the fdroiddata metadata, there is currently only 30 apps that do not set Current Version Code. So if people agree to this change, I'll add Current Version Code to all of those in fdroiddata, then make the change in `fdroidserver.