Restrict VPN Diagnostics callbacks to underlying networks.

ConnectivityDiagnosticsCallbacks should only be invoked for the
underlying networks declared by active VPNs. This encourages VPN apps to
declare their underlying networks.

The previous permission model for VPNs allowed active VPNs to receive
callbacks on any network.

Bug: 148903617
Test: atest FrameworksNetTests
Change-Id: Ic08cdd2e2532580fda0fd3034e2bdff27e0ff84b

services/core/java/com/android/server/ConnectivityService.java

@@ -7890,10 +7890,13 @@ public class ConnectivityService extends IConnectivityManager.Stub
             return false;
         }
 
+        final Network[] underlyingNetworks;
         synchronized (mVpns) {
-            if (getVpnIfOwner(callbackUid) != null) {
-                return true;
-            }
+            final Vpn vpn = getVpnIfOwner(callbackUid);
+            underlyingNetworks = (vpn == null) ? null : vpn.getUnderlyingNetworks();
+        }
+        if (underlyingNetworks != null) {
+            if (Arrays.asList(underlyingNetworks).contains(nai.network)) return true;
         }
 
         // Administrator UIDs also contains the Owner UID