Fix some typescript errors in //aos/network/www/...

The typescript compiler upgrade revealed these.

Change-Id: I882e6a93fe69cd45255ae7885b57467ac6cc7498
diff --git a/aos/network/www/ping_handler.ts b/aos/network/www/ping_handler.ts
index 9b37d70..a7fb0d4 100644
--- a/aos/network/www/ping_handler.ts
+++ b/aos/network/www/ping_handler.ts
@@ -4,8 +4,8 @@
   const fbBuffer = new flatbuffers.ByteBuffer(data);
   const ping = aos.examples.Ping.getRootAsPing(fbBuffer);
 
-  document.getElementById('val').innerHTML = ping.value();
-  document.getElementById('time').innerHTML = ping.sendTime().low;
+  document.getElementById('val').innerHTML = ping.value().toString();
+  document.getElementById('time').innerHTML = ping.sendTime().low.toString();
 }
 
 export function SetupDom() {