Iterate NetworkPolicy towards SubscriptionPlan.

Add new SubscriptionPlan API to describe the various types of carrier
data plans.  Internally the OS will only use the first plan for
driving policy, but it will blindly plumb through the details for
Settings to display any secondary plans.

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.

Replace all data usage cycle calculation with new SubscriptionPlan
implementation, and retrofit large suite of existing tests to
exercise and verify the new logic.

Offer a debugging property that can be used to return "fake" plan
examples for testing.

Bug: 63391323
Test: bit FrameworksServicesTests:com.android.server.NetworkPolicyManagerServiceTest
Exempt-From-Owner-Approval: Bug 63673347
Change-Id: I889c653980eeb7887abdfa4f5b6986f35855ee6d