Android demo to use darknet model in ncnn.

Steps:

1. Use darknet2ncnn to convert darknet model to ncnn model.
2. Build xiangweizeng/ncnn for android.
3. Put model, ncnn header files and static lib into project.

Reference: https://github.com/chehongshu/ncnnforandroid_objectiondetection_Mobilenetssd

For vulkan support: https://github.com/nihui/ncnn-android-styletransfer

You can find another user-friendly tool at: https://github.com/Tencent/ncnn/tree/master/tools/darknet