[FileSystem] Fix ConvertFrom() base64 decode algorithm

fixed by: Leandro A. F. Pereira
[FileSystem] Add FileStream fstream support

Before this fix, FileStream used FileDescriptor to access its files.
We had some problems dealing with the position attribute, so we
decide to look how tizen deals with it. Tizen uses fstream to handle
the FileStream class, so as we do now.