Ok, I realize that pop stars are supposed to follow trends to their obvious conclusion and create blockbuster-sized singles out of whatever the zeitgeist happens to offer up at that particular moment. So why am I so annoyed with Christina Aguilera hunkering down with producer Switch and co-writer M.I.A. on this new track leaked off her Bionic album dropping next week? Maybe it’s because this track is going to burn a hole in my gray matter all summer long, no matter how inferior it is to just about everything else Switch and Maya have done in their collective careers. Or maybe, since seeing Xtina destroy even the nose-bleed seats singing “It’s A Man’s World” in tribute to James Brown at the Grammys a few years ago, I thought she had evolved passed cheap popdom. I mean, even the senior ladies …