Right now they are just stored on a class called
I agree with puneet, it's neither good nor bad. It depends on what you are doing with the API Urls.
Are you going to append them later with user input? If so I would suggest you keep them as global variables that way you can modify the API URL programatically as needed.
If the API Url are complete and will not need to be appended then putting them in the strings.xml would be fine. Just remember that you would still have to create a local String variable in the java to hold the text from the API_URL in the string.xml, which seems inefficient if what you're aiming for is to write less code.