Branded drug companies’ share of total U.S. spending on their products is declining, in part because of an increase in secret rebates paid to middlemen, according to an analysis sponsored by an industry group that’s been seeking to deflect scrutiny of rising prices. Of the estimated $349.1 billion that insurers and patients paid for brand-name drugs in 2015, $218.6 billion, or 63 percent, was realized as revenue by drugmakers, according to an analysis paid for by the Washington drug lobby, Pharmaceutical Research and Manufacturers of America.