[SP02] Add provider interfaces to system API

This change contains a base class that allows external modules
to implement a custom network statistics provider. And a callback
interface that allows the implementation to signal the system.

These interfaces will be used in the follow-up changes that
implement the functionality.

Test: atest FrameworksNetTests CtsUsageStatsTestCases
Test: m doc-comment-check-docs
Bug: 130855321
Change-Id: Ib23377c8b9cef02bc32253462b068fd10734d21a