HarmonyOS 文件管理fs接口定位定界指导

保存沙箱下文件到公共目录参考示例：

writeFile file:{
  let documentSaveOptions = new picker.DocumentSaveOptions();
  documentSaveOptions.pickerMode = picker.DocumentPickerMode.DOWNLOAD
  documentSaveOptions.newFileNames = ['hilog.zip'];
  let documentPicker = new picker.DocumentViewPicker(context);
  documentPicker.save(documentSaveOptions).then((documentSaveResult: Array<string>) => {
    let srcPath = getContext().resourceDir + '/hilog.zip';
    let uriString = new fileUri.FileUri(documentSaveResult[0])
    let dstPath = uriString.path + "/hilog.zip";
    fs.copyFileSync(srcPath, dstPath);
    console.info('DocumentViewPicker.save successfully, documentSaveResult uri: ' + JSON.stringify(documentSaveResult));
  }).catch((err: BusinessError) => {
    console.error('DocumentViewPicker.save failed with err: ' + JSON.stringify(err));
  });
}