I believe sunset is usually around 6pm or shortly after. So not sure if you will get to the Galle Face by sunset, especially if you hit traffic from the airport to the city. Also, if you are going to be walking around the city or the greens, you would have to do something about your hand luggage. I believe there is a left luggage at CMB, but I have never used it.

I assume you are planning to have dinner at one of the higher end restaurants or tourist hotels in the Galle Face Greens area, and not have street food? Even I, who lived the first 20 years of my life in Sri Lanka, would be a little hesitant to eat street food after all these years .

The better thing to do would be to have dinner at on of restaurants in one of the hotels in the Negombo area, which is very close to the airport. There are several Jetwing hotels in the area and they do have pretty good restaurants. I have eaten at some of them, but I don't remember the names. If you look up tripadvisor you should be able to find some good places to eat in Negombo.