[FUI17] Migrate NetworkStatsService to use NetworkStateSnapshot

This change migrates NetworkStatsService and related code to use
NetworkStateSnapshot which is used for replacing the NetworkState.

This patch also changes some formating which is suggested by
the linter.

Test: FrameworksNetTests NetworkPolicyManagerServiceTest
Bug: 174123988
Change-Id: I547da8f411cb45bdadc376ac3cadf3f3c55bb282