Gradle structure

As described on https://f-droid.org/forums/topic/gradle-directory-structure/.

This will keep things a bit weird with git for a bit, but you can always use --follow to make it take renames into account for the commit history.

I tweaked both the ant and gradle build setups and both of them work in the new structure. And gradle is much faster since e.g. now an 'assemble' inside F-Droid assembles only the client, without including all the subprojects as it happens in master.