NsdManager: add unit tests

This prepares some refactoring and the addition of a timeout to
resolveService.

Test: new tests pass
Bug: 37013369, 33298084
Change-Id: Ie8277bd5983278507bfa70495b4ce7d13895b24b

core/java/android/net/nsd/NsdManager.java

@@ -31,6 +31,7 @@ import android.util.SparseArray;
 
 import java.util.concurrent.CountDownLatch;
 
+import com.android.internal.annotations.VisibleForTesting;
 import com.android.internal.util.AsyncChannel;
 import com.android.internal.util.Protocol;
 
@@ -326,7 +327,8 @@ public final class NsdManager {
         public void onServiceResolved(NsdServiceInfo serviceInfo);
     }
 
-    private class ServiceHandler extends Handler {
+    @VisibleForTesting
+    class ServiceHandler extends Handler {
         ServiceHandler(Looper looper) {
             super(looper);
         }