Move list of SystemUI services to xml

Previously the services of SystemUI were defined in SystemUIApplication.

This patch moves the service list to res/values/config.xml. So that it
gets easier to customize services of SystemUI.

Bug: 63874929
Bug: 62602530
Test: Ran and confirmed SystemUI started without error

Change-Id: I7f0321c46e562288b2cad5201ae120c37e853908