Stats - Remove ThreadPoolExecutor and use Volley threading capabilities

Fix #1199 - For a detailed description see the ticket.

With this patch all the network calls are enqueued in Volley at the beginning of the service; Volley takes care of handling multithreading. UI refresh is done by using a ThreadPoolExecutor with a single thread.
This way the time spent in the "network layer" is nearly the half of before.