Don't check broadcast intent on connection status check

testRequestNetworkCallback_onUnavailable is flaky because
the test expects a connectivity broadcast to be sent for the
lost network. But after testSetAirplaneMode, the wifi network
is not yet default network so a broadcast is never sent but
only onUnavailable is triggered. Replace disconnectFromWifi
with ensureWifiDisconnected to skip checking the broadcast.

Bug: 162323152
Test: atest CtsNetTestCasesLatestSdk:ConnectivityManagerTest\
      --rerun-until-failure 20
Change-Id: I88f434798ec83539df9cc6a81446ca37a0081e3b

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

@@ -607,7 +607,7 @@ public class ConnectivityManagerTest {
     public void testRequestNetworkCallback_onUnavailable() {
         final boolean previousWifiEnabledState = mWifiManager.isWifiEnabled();
         if (previousWifiEnabledState) {
-            mCtsNetUtils.disconnectFromWifi(null);
+            mCtsNetUtils.ensureWifiDisconnected(null);
         }
 
         final TestNetworkCallback callback = new TestNetworkCallback();