Keep all native libs in "jni" folder for AAR artifacts

Native libs are always kept in "jni" folder in AAR file. For legacy AAR artifacts native libs are moved from "libs" to "jni" folder when unpacking the artifact. Everything stays the same for APKLIB artifacts.

This is one more piece of the puzzle to provide complete support for AAR format, in accordance with its suggested internal layout. Read #402 for more details about it.
We should be consistent. Can you update this and ApkMojo so they are both referenced the same way (ie ApkMojo.NATIVE_LIBRARIES_FOLDER).  We might want to replace the "jni" and "libs" strings below with those constants as well.