Minor changes to LinkAddress.

1. Simplify the parceling code. Since the InetAddress inside a
   LinkAddress can never be null, we don't need to special-case
   the case where it is.
2. Add / update method documentation.
3. Write a unit test.

Change-Id: Iba0a8cecc683d55d736419965e72ee33dd66dc22
Merge "Minor changes to LinkAddress."