NDK mojo: ignore libs/ folder with just JARs for APKLIB dependencies

This should fix https://code.google.com/p/maven-android-plugin/issues/detail?id=402

Can someone review please? This was a show stopper for us, since it broke all NDK builds with apklib dependencies with libs/ folders containing JARs (which is perfectly fine to have, libs/ is not exclusive to native libraries.)

What this change accomplishes is simply ignoring the libs/ folder whenever it only contains JAR files.

I couldn't actually test this with a project that actually has an apklib dependency with native libraries in libs/. Would be good if someone who needs that functionality can verify it's still working.