[MS40.1] Remove Clock dependencies from NetworkStatsService

This includes:
1. Re-implement the BestClock in the static library.
2. Expose SystemClock#currentNetworkTimeClock as public Api

Test: TH
Bug: 204830222
CTS-Coverage-Bug: 213393821
Exempt-From-Owner-Approval: 1. Owner approved the change with
  explicitly granted submission after adderessing the
  straight-forward comment. 2. Owner is OOO for 3 months.
Change-Id: I08c0162a7101cc6184f3eca3d4a59211183fb843