Iterate NetworkPolicy towards SubscriptionPlan.

As part of getting Settings ready to roll towards SubscriptionPlan,
reduce references to NetworkPolicy internal fields.  All usage cycle
details are now obtained from an Iterator which converts to
SubscriptionPlan under the hood.

Bug: 63391323
Test: bit FrameworksServicesTests:com.android.server.NetworkPolicyManagerServiceTest
Exempt-From-Owner-Approval: Bug 63673347
Change-Id: I0e1357eef092a328c05f6a709aba0f106554631e