Collect and persist tethering stats.

Use new "gettetherstats" netd command to retrieve statistics for
active tethering connections.  Keep tethering poll events separate
from UID poll, even though they end up same historical structures.

Bug: 5244846
Change-Id: Ia0c5165f6712c12b51586f86c331a2aad4ad6afb

services/java/com/android/server/ConnectivityService.java

@@ -2394,6 +2394,12 @@ public class ConnectivityService extends IConnectivityManager.Stub {
         return mTethering.getTetheredIfaces();
     }
 
+    @Override
+    public String[] getTetheredIfacePairs() {
+        enforceTetherAccessPermission();
+        return mTethering.getTetheredIfacePairs();
+    }
+
     public String[] getTetheringErroredIfaces() {
         enforceTetherAccessPermission();
         return mTethering.getErroredIfaces();