Change AppPrefLoader from AsyncTask to AsyncLoader.

Showing the wifi data usage results in reaching the max number of
threads in the pool executor. Switching the loader implementation
from Async task to AsynLoader and use a single loader to get the
whole list of preferences instead of using a new task to create each
list element.

Change-Id: I0da19ca09031a7fa178970c192e12a99ab489145
Fix: 33820327
Test: make RunSettingsRoboTests