There are two Georgia O’Keeffes. They’re closely related, but one is far more interesting than the other. Not so interesting, except maybe as a marketing phenomenon, is the post-1930s cow-skull painter and striker of frontier-priestess poses. More interesting, and less familiar, is the artist found in “Georgia O’Keeffe: Abstraction,” a vivid and surprisingly surprising show of more than 130 paintings and drawings at the Whitney Museum of American Art.