To support skip464xlat per Network

To add skip464exlat in NetworkMisc.
NetworkAgent can skip to start 464xlat if need.
(e.g. IMS PDN for Cellular can be disabled)

Device will treat the network as IPv6-only if it is set

Bug: 69949375
Test: Nat464XlatTest, ConnectivityServiceTest

Change-Id: I676a02cb92530d64f29f34e89482a934f3ec4553