Apple Inc.’s iPhone, a new model of which is widely expected this summer, took 19.2 percent of the U.S. market for “smart” phones in the first quarter of 2008, according to research firm IDC’s vendor survey. That was down from 26.7 percent of smart phones sold in the fourth quarter of last year, which included the holiday shopping season, IDC found. Much of the slack was picked up by Research In …