[path_provider] Migrate off getExternalStorageDirectory #35299
Description
getExternalStorageDirectory is deprecated and is returning an invalid path when the app is targeting Android Q. The deprecation notice:
This method was deprecated in API level 29.
To improve user privacy, direct access to shared/external storage devices is deprecated. When an app targets Build.VERSION_CODES.Q, the path returned from this method is no longer directly accessible to apps. Apps can continue to access content stored on shared/external storage by migrating to alternatives such as Context#getExternalFilesDir(String), MediaStore, or Intent#ACTION_OPEN_DOCUMENT.
This method is currently used by the path_provider plugin (code).
We should migrate the plugin off this method.