wgengine/router: default to a fake router on android

The regular android app constructs its own wgengine with
additional FFI shims, so this default codepath only affects
other handcrafted buids like tsnet, which do not let the
caller customize the innards of wgengine.

Android >=14 forbids the use of netlink sockets, which makes
the standard linux router fail to initialize.

Fixes #9836

Signed-off-by: David Anderson <dave@tailscale.com>

wgengine/router/router_linux.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !android
+
 package router
 
 import (