Move battery stats loading to AsyncLoader.

It takes 3-4 seconds to load the battery settings page in Ryu. Main
reason is that it takes too long to init BatteryStatsHelper.

This cl moves loading code to AsyncLoader, and we will refresh ui once
the loading part is finished.

Following cl will add animation for the battery header to make it not
so janky

Bug: 37196170
Test: RunSettingsRoboTest
Change-Id: I40dfdde4a072e28a56c8fdf0ec6d671b5109fc6d