$args Orderby The number

You could use the get_terms_orderby filter:

add_filter( 'get_terms_orderby', 'wpse156153_get_terms_orderby', 10, 3 );
$categories=get_categories($cat_args);
remove_filter( 'get_terms_orderby', 'wpse156153_get_terms_orderby', 10, 3 );

with

function wpse156153_get_terms_orderby($orderby, $args, $taxonomies ) {
    return 'FIELD(t.term_id, ' . $args['include'] . ')';
}