Exactly a year after it was launched in the United States, the Sony PlayStation Portable’s days as a hand-held movie-viewing device might be numbered. Disappointing sales have slowed the flow of movies on the proprietary Universal Media Disc to a mere trickle. At least two major studios have completely stopped releasing movies on UMD, while others are either toying with the idea or drastically cutting back. And retailers also are cutting the amount of shelf space they’ve been devoting to UMD movies, amid talk that Wal-Mart is about to dump the category entirely.