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/netutil/interfaces_android.go

@@ -0,0 +1,21 @@
+// Copyright (C) 2025 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package netutil
+
+import (
+	"net"
+
+	"github.com/wlynxg/anet"
+)
+
+func Interfaces() ([]net.Interface, error) {
+	return anet.Interfaces()
+}
+
+func InterfaceAddrsByInterface(intf *net.Interface) ([]net.Addr, error) {
+	return anet.InterfaceAddrsByInterface(intf)
+}