fix(pmp, netutil): workaround native code denied to discover gateway ipv4 addr on Android 14+ (#10204)

### Purpose

As discussed on the forum:
https://forum.syncthing.net/t/reviving-nat-pmp-in-v2-x-on-android-14/24554

TL;DR
Android 14+ only lets java code get the gateway IPv4 address which is
used in SyncthingNative’s NAT-PMP feature.

So I’ve added the java code to the wrapper, got the router IP address
and feeded it to SyncthingNative by setting the env var
“FALLBACK_NET_GATEWAY_IPV4”.

This revives the NAT feature:

> [Z36WU] INFO: Detected 1 NAT service

### Testing

Local build and test via Android emulator (AVD 15).

lib/netutil/netutil.go

@@ -6,7 +6,14 @@
 
 package netutil
 
-import "net/url"
+import (
+	"fmt"
+	"net"
+	"net/url"
+	"os"
+
+	"github.com/jackpal/gateway"
+)
 
 // Address constructs a URL from the given network and hostname.
 func AddressURL(network, host string) string {
@@ -16,3 +23,22 @@ func AddressURL(network, host string) string {
 	}
 	return u.String()
 }
+
+// Gateway returns the IP address of the default network gateway.
+func Gateway() (ip net.IP, err error) {
+	ip, err = gateway.DiscoverGateway()
+	if err != nil {
+		// Fails on Android 14+ due to permission denied error when reading
+		// /proc/net/route. The wrapper may give a hint then because it is
+		// able to discover the gateway from java code.
+		if v := os.Getenv("FALLBACK_NET_GATEWAY_IPV4"); v != "" {
+			ip = net.ParseIP(v)
+			if ip == nil {
+				return nil, fmt.Errorf("%q: invalid IP", v)
+			}
+			return ip, nil
+		}
+		return ip, err
+	}
+	return ip, nil
+}

lib/pmp/pmp.go

@@ -14,10 +14,10 @@ import (
 	"strings"
 	"time"
 
-	"github.com/jackpal/gateway"
 	natpmp "github.com/jackpal/go-nat-pmp"
 
 	"github.com/syncthing/syncthing/lib/nat"
+	"github.com/syncthing/syncthing/lib/netutil"
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/svcutil"
 )
@@ -30,7 +30,7 @@ func Discover(ctx context.Context, renewal, timeout time.Duration) []nat.Device
 	var ip net.IP
 	err := svcutil.CallWithContext(ctx, func() error {
 		var err error
-		ip, err = gateway.DiscoverGateway()
+		ip, err = netutil.Gateway()
 		return err
 	})
 	if err != nil {