Change RequestTracker log tag to TetheringRequestTracker

So related logs show up when grep'ing for tethering.

Test: TH
Bug: 10000001
Flag: EXEMPT trivial
Change-Id: I86791be62a068a8a64eca35fd5ce01ecde750a04

Tethering/src/com/android/networkstack/tethering/RequestTracker.java

@@ -43,7 +43,7 @@ import java.util.Map;
  * Note: This class is not thread-safe.
  */
 public class RequestTracker {
-    private static final String TAG = RequestTracker.class.getSimpleName();
+    private static final String TAG = "TetheringRequestTracker";
 
     @NonNull
     private final boolean mUseFuzzyMatching;