The simple answer is India. But you may like some historical prospective
Chicken itself is native of India. There is evidence to suggest that Chicken existed in Vietnan nearly 10,000 years ago. Chicken has also existed in Thailand for over 2,000 years.. This does not answer you about chicken curry.
In the olden days chicken was used as a pet, show-bird, or for cock-fighting. Chicken eats insects bugs and crawlers. Such a meat was prohibited by most of the religions.
During the times of Budha, peacocks were popular in the royal kitchens. Still no chicken, The whole peacocks were stuffed with minced deer meat and roasted on an open fire.
During 1500s, the Hindu cooks in the royal kitchen of Akbar introduced Murgh Mussallum, the first documented recipe for chicken. It was same as the peacocks cooked during Asoka. The whole chicken was stuffed with spiced minced goatmeat and slowly roasted over open fire for hours. This dish is still prepared and served in many North Indain restaurants. But it is really not a curry.
In India sheep and goat meats were commonly used to make curry. In Asia, pork was a popular meat. Than came the British.
British were used to jackrabbit. They used stuffed pecocks for Christmas. The chicken meat was comparable to rabbit for British taste and more readily avaolable than the jackrabbits. British took the chicken to England and started to make chicken stew. This will become the foundation for Chicken curry.
The first chicken curry is called 'Country Captain Chicken' and was documented on April 26, 1792 in Madras Courier in India. In 1825, it would reach Georgia (USA) and would become a popular dish with President Roosevelt and General Patton
Captains chicken wil lead to various types of chicken curries. Chicken curry would take another turn in 1947. Jawahar Lal Nehru beomes the first prime minister of India. Chicken cuury was Nehru's favorite meat dish in UK. In Delhi, his favorite restaurant was Moti Mahal. They created the Buttery chicken and Chicken Tandoori for Nehru. These two dishes were served to all foreign heads of states who would visit Delhi. Buttery Chicken curry and Tandoori chicken would become two most popular Indian chicken dishes and served at all almost all the Induan restaurants.