appc: setting AdvertiseRoutes explicitly discards app connector routes

This fixes bugs where after using the cli to set AdvertiseRoutes users
were finding that they had to restart tailscaled before the app
connector would advertise previously learned routes again. And seems
more in line with user expectations.

Fixes #11006
Signed-off-by: Fran Bull <fran@tailscale.com>

ipn/localapi/localapi.go

@@ -1366,6 +1366,12 @@ func (h *Handler) servePrefs(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, err.Error(), http.StatusBadRequest)
 			return
 		}
+		if err := h.b.MaybeClearAppConnector(mp); err != nil {
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusInternalServerError)
+			json.NewEncoder(w).Encode(resJSON{Error: err.Error()})
+			return
+		}
 		var err error
 		prefs, err = h.b.EditPrefs(mp)
 		if err != nil {