Block incoming packets in VPN Lockdown mode.

Currently, even when VPN Lockdown mode is enabled, incoming packets are
not dropped if VPN is not connected.

This commit fixed this issue.
After this commit, If VPN Lockdown mode is enabled, incoming packets
are dropped regardless of the VPN connectivity.

Bug: 206482423
Test: atest TrafficControllerTest ConnectivityServiceTest PermissionMonitorTest
Change-Id: If52ece613c8aac1073355e43b6fb9cb3fcc87d1d

framework/src/android/net/ConnectivityManager.java

@@ -982,6 +982,16 @@ public class ConnectivityManager {
     @SystemApi(client = MODULE_LIBRARIES)
     public static final int FIREWALL_CHAIN_LOW_POWER_STANDBY = 5;
 
+    /**
+     * Firewall chain used for lockdown VPN.
+     * Denylist of apps that cannot receive incoming packets except on loopback because they are
+     * subject to an always-on VPN which is not currently connected.
+     *
+     * @see #BLOCKED_REASON_LOCKDOWN_VPN
+     * @hide
+     */
+    public static final int FIREWALL_CHAIN_LOCKDOWN_VPN = 6;
+
     /** @hide */
     @Retention(RetentionPolicy.SOURCE)
     @IntDef(flag = false, prefix = "FIREWALL_CHAIN_", value = {
@@ -989,7 +999,8 @@ public class ConnectivityManager {
         FIREWALL_CHAIN_STANDBY,
         FIREWALL_CHAIN_POWERSAVE,
         FIREWALL_CHAIN_RESTRICTED,
-        FIREWALL_CHAIN_LOW_POWER_STANDBY
+        FIREWALL_CHAIN_LOW_POWER_STANDBY,
+        FIREWALL_CHAIN_LOCKDOWN_VPN
     })
     public @interface FirewallChain {}
     // LINT.ThenChange(packages/modules/Connectivity/service/native/include/Common.h)