Nearly a decade after she became the first American Idol, Kelly Clarkson’s principal artistic asset — beyond that fire hose of a voice, of course — remains her believability. Who else at this level could refer credibly to herself and an ex as “just a couple of kids trying to figure out how to live,” as she does on “I Forgive You”? Stronger isn’t Clarkson’s long-promised Nashville album; it mostly sticks to the bright-and-shiny bubble-grunge sound of “Since U Been Gone.” But she delivers tunes like “Einstein” and the disco-glam title track with a country singer’s earthy conviction. She’s not Katy Perry, not yet Carrie Underwood. But look out.