Issue/976 optimize stats

Fix #976 - stats have been optimized for performance to prevent the ANRs we've seen in crash reports. Changes include:

* Replaced the AsyncTask-based background updating with an Android service that employs a ThreadPoolExecutor for concurrent updating
* Split the various REST calls into separate runnable tasks for use with the above service
* Rewrote lists to use view recycling & the holder pattern
* Simplified layouts of group/child items
* Skipping parsing & caching of excess stats data that never gets displayed
* Skipping unnecessary data reloading & view refreshing
* Moved processing of stats data to separate threads prior to display
* Numerous other tweaks that I've already forgotten about :)