I don't know anyone that doesn't do this, it's not odd to me in the slightest. I do something a little similar with my watch, sometimes it's just aggravating on the skin.
A lot of people have these awful fidgeting habbits. I have them too. I always play with my rings and roll them always did that's nothing to do with Elvis. People are doing these kind of things either because their nervous or just fidgety in general. Like me. My wife keeps telling me to stop that and biting my fingernails too all the time. I think in Elvis' case he got it from his father of course just like somebody said up there.
Elvis also used to put his tongue out on occasion, usually when in a playful mood. I'm sure we all have unusual mannerisms but don't know it. It usually takes other people to recognise these.
I use to tug on my left ear as a child, I also have noticed (well my wife noticed and pointed it out to me) that my body and are movements are exaggerated when I'm nervous or in an uncomfortable situation....just odd human traits that are peculiar to each person for some reason or other that takes therapy to figure out and even then, you still don't always know why.:D
I think everyone has some little habit they do in certain situations. I just always wondered about Elvis being so hyper in his younger years. There's no way of knowing for certain if he was ADHD unless we knew how short or long his attention span was and no one has every commented on that.