Handle "show touches" on the device-side

Now that the server can access the Android settings and clean up
properly, handle the "show touches" option from the server.

The initial state is now correctly restored, even on device
disconnection.

app/src/server.h

@@ -51,6 +51,7 @@ struct server_params {
     int8_t lock_video_orientation;
     bool control;
     uint16_t display_id;
+    bool show_touches;
 };
 
 // init default values