Track and persist in stats whether traffic is on the default network.

This change adds the defaultNetwork to the ident based on current
system networking state, and persists that value to flash.

Bug: 35142602
Test: runtest frameworks-net
Test: downloaded files using default/non-default networks while watching dumpsys netstats
Change-Id: Id8776d149b2977548a9eb455ad4926af55e25aba