refactor(beacon, osutil, upnp, netutil): only use anet on Android (#10211)

Add a wrapper that uses anet on Android, but net on other platforms.

### Purpose

Fixes
https://forum.syncthing.net/t/workaround-for-android-local-discovery/20403/12

### Testing

Run two Syncthing instances with Global Discovery disabled. Pair them
with each other, don't hardcode their addresses, and verify they
connect.

lib/beacon/broadcast.go

@@ -11,7 +11,7 @@ import (
 	"net"
 	"time"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 )
 
 func NewBroadcast(port int) Interface {
@@ -46,7 +46,7 @@ func writeBroadcasts(ctx context.Context, inbox <-chan []byte, port int) error {
 			return doneCtx.Err()
 		}
 
-		intfs, err := anet.Interfaces()
+		intfs, err := netutil.Interfaces()
 		if err != nil {
 			l.Debugln("Failed to list interfaces:", err)
 			// net.Interfaces() is broken on Android. see https://github.com/golang/go/issues/40569
@@ -61,7 +61,7 @@ func writeBroadcasts(ctx context.Context, inbox <-chan []byte, port int) error {
 				continue
 			}
 
-			addrs, err := anet.InterfaceAddrsByInterface(&intf)
+			addrs, err := netutil.InterfaceAddrsByInterface(&intf)
 			if err != nil {
 				l.Debugln("Failed to list interface addresses:", err)
 				// Interface discovery might work while retrieving the addresses doesn't. So log the error and carry on.

lib/beacon/multicast.go

@@ -12,7 +12,7 @@ import (
 	"net"
 	"time"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 
 	"golang.org/x/net/ipv6"
 )
@@ -61,7 +61,7 @@ func writeMulticasts(ctx context.Context, inbox <-chan []byte, addr string) erro
 			return doneCtx.Err()
 		}
 
-		intfs, err := anet.Interfaces()
+		intfs, err := netutil.Interfaces()
 		if err != nil {
 			l.Debugln(err)
 			return err
@@ -119,7 +119,7 @@ func readMulticasts(ctx context.Context, outbox chan<- recv, addr string) error
 		conn.Close()
 	}()
 
-	intfs, err := anet.Interfaces()
+	intfs, err := netutil.Interfaces()
 	if err != nil {
 		l.Debugln(err)
 		return err