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/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 {