Suffolk Downs will dramatically shrink the “sea of asphalt” around the East Boston racetrack and increase plantings and gardens, as a key selling point of its bid for casino development rights, track officials say. The landscaping plan is the first major update to the Suffolk Downs casino design since the track initially rolled out plans for a $1 billion “urban oasis’’ last June. The update come as Suffolk Downs, once the unchallenged front-runner for the state’s most lucrative casino license, is facing tough new competition from rival projects.