Make LinkPropertiesTest pass, and fix an NPE.

aosp/973686 added the DHCP server address to LinkProperties, but
it did not have any tests, and would throw NPE when parceling a
LinkProperties that did not have a DHCP server.

Add tests and fix the bug.

Bug: 134098566
Test: unit test now passes
Change-Id: If28f0be650963e8d9af7eb53f6940824582a2db3