And the battle of the singing shows continues! After “American Idol” announced its all-star lineup this week featuring Mariah Carey, Nicki Minaj and Keith Urban, their rival show, NBC’s “The Voice” followed suit with a major announcement of their own. NBC announced that Usher and Shakira will be joining “The Voice” as new coaches next spring when Christina Aguilera and Cee-Lo Green take breaks from the show to focus on their careers. The network said original coaches Adam Levine and Blake Shelton will remain on...