Cover corn husks with cold water; soak until softened, at least 2 hours., Place chicken, onion, salt and garlic in a 6-qt. stockpot. Pour in 3 qt. water; bring to a boil. Reduce heat; simmer, covered, until chicken is tender, 45-60 minutes. Remove chicken from broth. When cool enough to handle, remove bones and skin; discard. Shred chicken. Strain cooking juices; skim fat. Reserve 6 cups stock., For dough, beat shortening until light and fluffy, about 1 minute. Beat in small amounts of masa harina alternately with small amounts of reserved stock, using no more than 2 cups stock. Drop a small amount of dough into a cup of cold water; dough should float. If not, continue beating, rechecking every 1-2 minutes., For filling, heat oil in a Dutch oven; stir in flour until blended. Cook and stir over medium heat until lightly browned, 7-9 minutes. Stir in seasonings, chicken and remaining stock; bring to a boil. Reduce heat; simmer, uncovered, stirring occasionally, until thickened, about 45 minutes., Drain corn husks and pat dry; tear 4 husks to make 20 strips for tying tamales. (To prevent husks from drying out, cover with a damp towel until ready to use.) On wide end of each remaining husk, spread 3 tablespoons dough to within 1/2 in. of side edges; top each with 2 tablespoons chicken filling and 2 teaspoons olives. Fold long sides of husk over filling, overlapping slightly. Fold over narrow end of husk; tie with a strip of husk to secure., Place a large steamer basket in the stockpot over water; place tamales upright in steamer. Bring to a boil; steam, covered, adding hot water as needed, until dough peels away from husk, about 45 minutes.