LOS ANGELES — Walt Disney postponed the debut of live-action movie “Mulan” until Aug. 21, the company said in a statement on Friday, a blow to theater operators who want blockbuster movies to draw audiences from their homes in the middle of a pandemic.“Mulan” was scheduled to debut in March but was postponed until July 24 when the coronavirus outbreak forced theaters around the world to close.“While the pandemic has changed our release plans for ‘Mulan’ and we will continue to be flexible as conditions require, it has not changed our belief in the power of this film and its message of hope and perseverance,” Alan Horn and Alan Bergman, co-chairmen of Walt Disney Studios, said in a statement.“Mulan” is a $200-million live-action remake of