Update Instructions on Native Android Build

1. Currently, `brew install android-ndk` installs NDK `r13b`, which is not compatible to current version of devkit. I have to manually download version `r9b` and set it up in PATH environment to make the build working

2. On macOS (I am using El Capitan), Java and Ant must be installed as well to make the build working:

```
brew cask install java
brew install ant
```
 