[DU04]Remove NetworkPolicyManagerInternal from NetworkStatsService

NetworkStatsService is using NetworkPolicyManagerInternal APIs, which
cannot be accessed after moving into the mainline module. So, replace
and remove those hidden API usages.

Bug: 211843446
Test: atest NetworkStatsServiceTest NetworkPolicyManagerTest
            NetworkPolicyManagerServiceTest
Change-Id: I0f3f02d1dc014967fab27ffcf37dd1918dbef5e0
7 files changed