This allows us to maintain NetworkStats entries that track
whether the traffic was on the default network.
At the moment, the stats collection code always passes in
DEFAULT_NETWORK_NO. However, this value is a no-op, since it is
not persisted to disk. Only the ident, the uid/set/tag, and the
packet/byte/operation counters are persisted.
A future change will add defaultNetwork to the ident and start
persisting it.
Bug: 35142602
Test: runtest frameworks-net
Change-Id: Ifa291c62c0fa389b88e5561086a29dcd7cee2253
3e85855083