Handle externalstorage.documents in getRealPathFromURI

[Airyzz]

Related to #365

I doubt this is really a recommended way to do things, but it's okay for my use case and figured I would share in case others find it useful.

[RonRadtke]

Did you test if it works with normalizing the path as it does in other locations?

[ClaudiuHBann]

This patch works, but in my case the method PathResolver::getRealPathFromURI fails for this branch:

if (isKitKat && DocumentsContract.isDocumentUri(context, uri)) {
    ...
    if (isExternalStorageDocument(uri)) {
        ...
        if ("primary".equalsIgnoreCase(type)) {
            // this will return the package's directory and not the external storage directory
            File dir = context.getExternalFilesDir(null);

and this must be changed to File dir = Environment.getExternalStorageDirectory(); aswell.