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

core/java/android/net/IConnectivityManager.aidl

@@ -83,6 +83,12 @@ interface IConnectivityManager
 
     String[] getTetheredIfaces();
 
+    /**
+     * Return list of interface pairs that are actively tethered.  Even indexes are
+     * remote interface, and odd indexes are corresponding local interfaces.
+     */
+    String[] getTetheredIfacePairs();
+
     String[] getTetheringErroredIfaces();
 
     String[] getTetherableUsbRegexs();

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();