Refactor name unpack_directory()

Refactor name unpack_directory()

The method unpack_directory() just returns the unpack_directory string,
    but its name implies for class users to "Unpack the directory".
    This commit changes the name from unpack_directory to
    unpacked_dir_path, of the getter, setter, and private variables.

Change-Id: I7259bbb3c8630d7464a98707162d955de45c9432