[ flutter ]dart.io 접근 함수

1. getApplicationDocumentsDirectory() : 해당 앱에서만 액세스 할 수 있는 지정된 경로
  => /data/user/0/kr.co.findplay.app/app_flutter
2. getApplicationSupportDirectory()
  => /data/user/0/kr.co.findplay.app/files
3. getExternalCacheDirectories()
  => [Directory: '/storage/emulated/0/Android/data/kr.co.findplay.app/cache']  
4. getExternalStorageDirectories({StorageDirectory type })
  => ???
5. getExternalStorageDirectory() : 해당 앱과 사용자가 액세스 할 수 잇는 지정된 경로
  => /storage/emulated/0/Android/data/kr.co.findplay.app/files
6. getLibraryDirectory()
  => null
7. getTemporaryDirectory()
  => Directory: '/data/user/0/kr.co.findplay.app/cache'