Chicken Adobo With Chicken Livers

Ingredients:
2 tbsp. vegetable oil, 1 tbsp. annatto seeds, 3 lb. whole chicken legs, separated into drumsticks and thighs, 4 cups distilled white vinegar, 1 tbsp. kosher salt, plus more, 2 tsp. whole black peppercorns, 3 bay leaves, 6 garlic cloves, thinly sliced, 1 large yellow onion, halved and cut lengthwise into 1/2-inch-thick slices, 1 lb. chicken livers, rinsed and dried