NsdService: test coverage for client requests.

Adding coverage for:
  - NsdManager client disconnection
  - in-flight request GC

Test: new test passes
Bug: 37013369, 33298084
Bug: 38503832

(cherry picked from commit 4dd4db7720)

Change-Id: I02aca772d88306acae09db739ced7a2b1119b72b

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

@@ -269,6 +269,14 @@ public final class NsdManager {
         init();
     }
 
+    /**
+     * @hide
+     */
+    @VisibleForTesting
+    public void disconnect() {
+        mAsyncChannel.disconnect();
+    }
+
     /**
      * Failures are passed with {@link RegistrationListener#onRegistrationFailed},
      * {@link RegistrationListener#onUnregistrationFailed},