Android RNFS.downloadFile only store file to root access directory

### Environment
react: ^16.3.1
react-native: ^0.56.0
react-native-fs: ^2.11.18

### Current behaviour
on android set the download file path to`RNFS.DocumentDirectoryPath`, but unfortunately it will only store to android root access directory `/data/user/0/com.[appname]/files`.

```
const ret = RNFS.downloadFile({
fromUrl: url,
toFile: RNFS.DocumentDirectoryPath + file.jpg,
});
```

any way i can save the downloaded file to public directory allow user to access from gallery?