LOS ANGELES (Reuters) - Britney Spears showed off her mean streak as the new talent judge on "The X Factor," but the TV singing contest stumbled with American viewers, drawing a smaller audience than last year and getting drowned out by "The Voice" and "America's Got Talent." In a first head-to-head battle of three talent shows, an average 8.5 million Americans - about 3.5 million or 32 percent ...