[FUI09] Expose NetworkStateSnapshot as system API

This is needed by NetworkStatsService and
NetworkPolicyManagerService to utilize the snapshots of
information of networks that grabbed from ConnectivityService.

Test: atest FrameworksNetTests NetworkPolicyManagerServiceTest
Bug: 174123988
Change-Id: I7e974ef7b23ba9ba6ee775eed9899b0c0e2eca55