[DU04-1]Remove NetworkPolicyManagerInternal from NetworkStatsService

Expose system APIs for use by NetworkStatsService.

Bug: 211843446
Test: atest NetworkStatsServiceTest NetworkPolicyManagerTest
            NetworkPolicyManagerServiceTest
Change-Id: I56a066b7f4b4c2c8fefdd185c2e50de63b1e2629
6 files changed