Always add maven support repository

If the m2 support repo is added after dependencies are declared, then the dependencies won't resolve. This isn't an issue if the repo already exists, as Google's android gradle plugin takes care of it, but if it needed to be downloaded, this will cause the first fresh build to fail.

I modified the plugin to always add the m2 support repo instead. If it's not needed (and thus doesn't exist), it shouldn't hurt anything as gradle will simply treat it as an empty repository.