While numerous trade reports have indicated that advertisers are returning to the broadcast networks and that an economic turnaround for the television industry appears to be in progress, no such signs were evident in the Walt Disney Co.' s latest quarterly report. The company said that its ABC network and television production studio experienced a 24-percent drop in operating income to $123 million. It blamed the falloff on lower ad revenue and higher production costs. While revenue at its 10 owned-and-operated TV stations was up 20 percent from a year ago, the company said, revenue remained well below 2008 levels