I wouldn't worry too much about whether you'll be social enough at your internship. I agree that people are generally very welcoming to summer interns. All you need to do is accept the lunch invitations, smile and say a few words, and you'll probably be remembered as a perfectly pleasant person. Combine that with some good hard work, and you'll be a success.
As for your legal career, it's true that you will need to forge at least a few strong relationships no matter where you end up working. I think finding the right setting is important. They say there are 3 kinds of law firm lawyers: finders, minders and grinders. Each role involves different types of relationships. If you love churning out the research and hate the idea of client cultivation and golf on Saturday mornings, then avoid a firm that wants all partners to be finders or minders and find one that has a place for grinders.
Also, it's vitally important to find yourself a mentor. I've managed to do this throughout my career even though I have a lot of trouble with social interactions. I've found that there's always someone who is happy to give advice -- you just need to ask. If it's someone you can do work for, all the better. Do good work for them and they'll go to bat for you.
As for practice areas, I don't think any particular one is "friendlier" for aspies. It really depends on where you work. That said, I think tax law is an awesone field for people who are very technical and like getting paid to do logic puzzles all day (yes, I'm a tax lawyer).
Good luck to you!