Add the default network to NetworkStats and NetworkStatsCollection.

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