Because we only need copy the target native library for target apk, such as: x86's libxwalkcore.so for x86's apk. So we need to separate the path for hosting libxwalkcore.so.

Originally, the directory hierarchy is native_libs/libs/x86 and native_libs/libs/armeabi-v7a,
and append the "native_libs/libs" to the "-DNATIVE_LIBS_DIR=". Then all the native libraries would 
include into the apk.

Now, with the above change, take x86 apk as example, only append the "native_libs/x86/libs" to the "-DNATIVE_LIBS_DIR=". Only the x86' library would be include into the apk. Because under the directory "native_libs/x86/libs" , there only exists "x86" directory for x86 library.