Always check off-link connectivity in NetworkDiagnostics.

Currently, NetworkDiagnostics only checks off-link connectivity if
one of the DNS servers is off-link. Make it check off-link
connectivity in all cases by sending probes to Google Public DNS
if off-link DNS servers are not specified.

Bug: 22569331
Bug: 22641669
Bug: 22748900
Change-Id: I6cb0dd8491bc0c1a488631deca56722b9c1d2b3f

core/java/android/net/LinkProperties.java

@@ -661,6 +661,17 @@ public final class LinkProperties implements Parcelable {
         return false;
     }
 
+    /**
+     * Returns true if this link or any of its stacked interfaces has an IPv4 address.
+     *
+     * @return {@code true} if there is an IPv4 address, {@code false} otherwise.
+     */
+    private boolean hasIPv4AddressOnInterface(String iface) {
+        return (mIfaceName.equals(iface) && hasIPv4Address()) ||
+                (iface != null && mStackedLinks.containsKey(iface) &&
+                        mStackedLinks.get(iface).hasIPv4Address());
+    }
+
     /**
      * Returns true if this link has a global preferred IPv6 address.
      *
@@ -792,7 +803,7 @@ public final class LinkProperties implements Parcelable {
 
         if (ip instanceof Inet4Address) {
             // For IPv4, it suffices for now to simply have any address.
-            return hasIPv4Address();
+            return hasIPv4AddressOnInterface(bestRoute.getInterface());
         } else if (ip instanceof Inet6Address) {
             if (ip.isLinkLocalAddress()) {
                 // For now, just make sure link-local destinations have