Noting that tablet computers like the iPad are increasingly the “first screen” for pre-school-age viewers, Disney executives said they would make the first nine episodes of a prominent new series available on mobile devices first. The series, “Sheriff Callie’s Wild West,” will arrive on the Watch Disney Junior app and a related Web site on Nov. 24. Then, early next year, the series will have its debut on two traditional TV networks, Disney Channel and Disney Junior.