[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
diff --git a/service-t/src/com/android/server/net/NetworkStatsService.java b/service-t/src/com/android/server/net/NetworkStatsService.java
index ef84ce0..5584293 100644
--- a/service-t/src/com/android/server/net/NetworkStatsService.java
+++ b/service-t/src/com/android/server/net/NetworkStatsService.java
@@ -113,7 +113,6 @@
 import android.net.netstats.provider.INetworkStatsProvider;
 import android.net.netstats.provider.INetworkStatsProviderCallback;
 import android.net.netstats.provider.NetworkStatsProvider;
-import android.os.BestClock;
 import android.os.Binder;
 import android.os.DropBoxManager;
 import android.os.Environment;
@@ -149,6 +148,7 @@
 import com.android.internal.annotations.GuardedBy;
 import com.android.internal.annotations.VisibleForTesting;
 import com.android.internal.util.FileRotator;
+import com.android.net.module.util.BestClock;
 import com.android.net.module.util.BinderUtils;
 import com.android.net.module.util.CollectionUtils;
 import com.android.net.module.util.NetworkStatsUtils;