[SP03] support registerNetworkStatsProvider API

This change provides an API that allow external modules to
register a custom provider of NetworkStats to merge the
network statistics that cannot be seen by the kernel to system.

Test: atest FrameworksNetTests CtsUsageStatsTestCases
Test: atest NetworkPolicyManagerServiceTest
Test: m doc-comment-check-docs
Bug: 130855321

Change-Id: I265bc637c40666cde505fde5056d2d9bfc5fb204