cmd/tailscale: disable smooth tunnel handovers for ChromeOS

ChromeOS doesn't seem to correctly apply new tunnel settings if
another tunnel is already active. Instead, close any existing
tunnel before creating another.

Updates tailscale/tailscale#431

Signed-off-by: Elias Naur <mail@eliasnaur.com>

cmd/tailscale/backend.go

@@ -122,6 +122,7 @@ func (b *backend) setCfg(cfg *router.Config) error {
 }
 
 func (b *backend) updateTUN(service jni.Object, cfg *router.Config) error {
+	b.CloseTUNs()
 	if reflect.DeepEqual(cfg, b.lastCfg) {
 		return nil
 	}