Fingernail biting can be seen as a form of cannibalism but since you're not eating your flesh which is a self cannibalism then I don't see it as cannibalism.
Instead of being cannibalism biting your nails is really Pica instead of cannibalism.
Pica is a disorder where you eat things that are not really edible such as hair, fingernails, paint chips etc.
So if you bite your fingernails it's more of the disorder pica.
If you begin eating your own skin then that would be cannibalism.