Move 'clearCollision' call from DcdCollision to CollisionPair

Move the responsibility of clear the colliding list of each collision
representation from DcdCollision::updatePair() to
CollisionPair::setRepresentations().

Make the logic more compact, given that in
Collision::setRepresentations(), clearContacts() will be called as well.
Move 'clearCollision()' from CollisionPair back to DcdCollision
Add 'isCollidingWith()' to check collision with given representation
Revise unit test based on comments and add more unit tests.