Change the url in testDownloadWithDownloadManagerDisallowed

In China, "https://www.google.com" will redirect to url
"http://www.google.cn" but cleartext traffic is default disabled
for an app. So the testDownloadWithDownloadManagerDisallowed
would failed by cleartext traffic is not permitted. Thus, change
the url in the test which is reachable in China.

Bug: 192978910
Test: atest HostsideVpnTests#testDownloadWithDownloadManagerDisallowed
Change-Id: I992c3240f0e133f5b00468cbe85d1545bcf786b0

tests/cts/hostside/app/src/com/android/cts/net/hostside/VpnTest.java

@@ -1213,7 +1213,8 @@ public class VpnTest extends InstrumentationTestCase {
                     new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
 
             // Enqueue a request and check only one download.
-            final long id = dm.enqueue(new Request(Uri.parse("https://www.google.com")));
+            final long id = dm.enqueue(new Request(
+                    Uri.parse("https://google-ipv6test.appspot.com/ip.js?fmt=text")));
             assertEquals(1, getTotalNumberDownloads(dm, new Query()));
             assertEquals(1, getTotalNumberDownloads(dm, new Query().setFilterById(id)));