Expose SubscriptionPlan API.

Clean up SubscriptionPlan, add docs, and expose as public API.

Deprecate older constants in CarrierConfigManager, and point users
towards new API.  (The goal of CarrierConfigManager is to express
key/value pairs that are identical for all users on a particular
carrier, not to configure details that vary between users.)

Since a CarrierService may itself not have access to the plan
details, allow them to "delegate" the capability to configure plans
to another package name.

Factor out new RecurrenceRule class which is used by both
NetworkPolicy and SubscriptionPlan internally.  Polish it to support
arbitrary periods, and continue to test it through NPMST.

Bug: 63391323
Test: bit FrameworksCoreTests:android.util.RecurrenceRuleTest
Test: bit FrameworksServicesTests:com.android.server.NetworkPolicyManagerServiceTest
Exempt-From-Owner-Approval: Bug 63673347
Change-Id: I93fae4a43e58e9e4c32f8bf345a257dcd607f088