[MS37] Replace NetworkStatsManagerInternal usages in NPMS

This is done by:
1. Add NetworkStatsManagerInternal APIs directly from
   NetworkStatsManager, these APIs are needed by NPMS.
2. Replace all usages with these APIs.
3. Delete NetworkStatsManagerInternal implementation.

Test: atest FrameworksNetTests NetworkPolicyManagerServiceTest
Bug: 204830222
CTS-Coverage-Bug: 213124616
Change-Id: If51b6676915e3a0a8a9f95221d735306911442fc