At a stop at the SAP Center in San Jose on Friday, the 28-year-old Barbadian singer, known for her racy Instragram posts and raunchy pop hits, opened the concert not with one of her many club bangers but the mournful piano ballad, “Stay,” as she stood draped in an oversized canvas hoodie atop a bare white platform in the middle of the arena floor. Rihanna, who made her appearance 30 minutes after her set’s scheduled start time, also made a vague reference to some backstage troubles but…