[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

core/api/current.txt

@@ -32074,6 +32074,7 @@ package android.os {
 
   public final class SystemClock {
     method @NonNull public static java.time.Clock currentGnssTimeClock();
+    method @NonNull public static java.time.Clock currentNetworkTimeClock();
     method public static long currentThreadTimeMillis();
     method public static long elapsedRealtime();
     method public static long elapsedRealtimeNanos();

core/java/android/os/SystemClock.java

@@ -311,7 +311,6 @@ public final class SystemClock {
      * time or throw.
      *
      * @throws DateTimeException when no accurate network time can be provided.
-     * @hide
      */
     public static @NonNull Clock currentNetworkTimeClock() {
         return new SimpleClock(ZoneOffset.UTC) {

packages/ConnectivityT/service/src/com/android/server/net/NetworkStatsService.java

@@ -113,7 +113,6 @@ import android.net.Uri;
 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 android.util.proto.ProtoOutputStream;
 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;