Split loading process for battery header

This cl creates BatteryHeaderPreferenceController, which will first
load the battery level in ui thread(in displayPreference()), then
display the remaining time once we get the battery stats.

This is the best we can do to avoid ui flashing while removing the
animation

Bug: 38349587
Test: RunSettingsRoboTests

Change-Id: Ic3622bb73562ba42e69b37e5674a724c03c45f12
5 files changed