Publish onionmasq for maven

@cyberta mentioned that having onionmasq published in MavenCentral would be good, this would let us avoid putting the .aar in the tpo/applications/vpn repository, and make it available for other people to use.

Additionally, we can also publish maven bits into the gitlab package repository here (see #50 (closed)) for that.

Maybe we should do both? The Maven Central for the world, and the gitlab package repository for our builds, for self-hosting reasons? I'm not too familiar with maven in general, so any thoughts, suggestions here would be greatly appreciated.