Looking at it, the libs/arch/.so files are just compiled JNI/NDK libraries compiled from source of jni/* files (that's completely standard for native code on Android). Usually the compiled libs aren't checked into the source control though.
Taking my paranoid glasses off, I'd guess they're checked in so contributors don't have to deal with setting up NDK to compile the app.
Taking my paranoid glasses off, I'd guess they're checked in so contributors don't have to deal with setting up NDK to compile the app.