Remove BaseNetworkObserver from Nat464Xlat

Let ConnectivityService talk to Nat464Xlat through NetworkAgnetInfo
directly:
- Nat464Xlat will be created once there is a new network bringing up,
but Netd doesn't expose the method for unregistering unsolicited event.
- ConnectivityService has already registered netd unsolicited event
listener, so Nat464Xlat doesn't need to register unsolicited event
listenser by itself, it can get the unsolicited event from
ConnectivityService directly.

Bug: 174837473
Test: atest FrameworksNetTests
Change-Id: I751526e8ffa7d25591696ecd8d93b1585c4c20c7