Make NduseroptMessage constructor visible for testing.

Refactor IpClientTest by parsing the netlink message directly instead of
netd callbacks, so we have to initialize an NduseroptMessage instance to
trigger IPv6 RDNSS option update in the unit test.

Bug: 318764217
Test: atest NetworkStackTests
Change-Id: Ic2e8ca0d5e71b73c0dbdc3311e72486af371c899