Unregister NetworkCallback in NetworkStatsManagerTest

NetworkStatsManagerTest issues NetworkRequests but does not release
them. In addition, it performs blocking operations inside the
onAvailable() callback that runs on the shared ConnectivityThread
impacting tests that run later.

Test: TH
Bug: 272147742
Change-Id: Ide2f5ba0b12752099d7665deaa9050463cd7ebee

tests/cts/net/src/android/net/cts/NetworkStatsManagerTest.java

@@ -387,6 +387,7 @@ public class NetworkStatsManagerTest {
                 now = System.currentTimeMillis();
             }
         }
+        mCm.unregisterNetworkCallback(callback);
         if (callback.success) {
             mNetworkInterfacesToTest[networkTypeIndex].setMetered(callback.metered);
             mNetworkInterfacesToTest[networkTypeIndex].setRoaming(callback.roaming);