Make RtNetlinkRouteMessage constructor visible for testing.

Refactor IpClientTest by parsing the netlink message directly instead
of netd callbacks, so we have to initialize a RtNetlinkRouteMessage
instance to trigger IPv4/v6 route updates in the unit test.

Bug: 318764217
Test: atest NetworkStackTests
Change-Id: Icfcc7ade636c89a63d897f4e9246410f6387b3c9