EAST RUTHERFORD, N.J.–Even in the 20-minute sets that Live Earth allots, acts are searching for the appropriately green cover song. The country singer Keith Urban found an improbable one: the Rolling Stones’ “Gimmie Shelter,” once a song about the solace of drugs and love and now about…the ozone layer? Regardless, he charged into it and shared it with none other than Alicia Keys, who’s been keeping a low profile lately. She belted it as if she’s been reborn a full-fledged rocker. (In South Africa, the same song was a duet for Angelique Kidjo and Joss Stone. )