A wildlife habitat--where 2,400 homes, a golf course and a commercial center had been planned--has been sold to the Nature Conservancy, a nonprofit environmental organization. In a complex financial deal, the group bought 3,825 acres of the Santa Rosa Plateau in Riverside County from developer RANPAC Inc. for more than $35 million, officials said. The organization already owns 3,100 acres of the plateau, home for such endangered species as the golden eagle, mountain lion and burrowing owl. On April 21, 1990, Earth Day, RANPAC agreed to negotiate with the conservancy and other agencies after its proposal to develop the property became the center of controversy. The purchase price includes $15.4 million from the Metropolitan Water District, $5 million in state bond money, $300,000 in land and other costs from the Conservancy and $15 million from Riverside County.