Fix LinkProperties's equals() method.

LinkProperties's equals() method was broken by the addition of
stacked interfaces. The reason was that equals() was checking
the equality of mStackedInterfaces.keys(), which is just an
enumeration, instead of mStackedInterfaces.keySet(), which
actually contains the keys. The test was failing, but I didn't
notice.

Fix the bug and make the test check the objects more in depth
so it can give more detailed error messages when equals() fails.

Bug: 8276725
Change-Id: Ie990bd75f641c28e63e54d953dcd0f4de13f7c9f