EAST RUTHERFORD, N.J. — Of all the Giants’ signings from their $200 million free agency splurge, perhaps none is as important as Adoree’ Jackson, whom the team expects to improve a secondary long yearning for a credible No. 2 cornerback. Deployed opposite the Pro Bowler James Bradberry, Jackson, who signed a three-year deal with $24.5 million guaranteed, should enable the Giants to play more man-to-man coverage, which they were reluctant to do last season.