Add getDefaultNetwork to the NetworkStats public API.

This allows an app using the public API to know whether the
traffic in a particular bucket was on the default network
(i.e., the network was selected by the system), or not (i.e.,
the network was selected by the app).

Bug: 35142602
Test: builds, boots
Test: added coverage to NetworkUsageStatsTest CTS test, still passes
Change-Id: I9f6669908fa119743b9c0aa0c31a03e5ebafa7db