Stop listening to config changes in AppWidgetService

Config chages are required to load the correct label. But
the label had been deprecated few years ago and hosts are
expected to load the label themselves
This avoids another reload just after unlock since the local
list is not prepared initially

Bug: 180867488
Test: Presubmit and CTS already passing
Change-Id: Iff4543bb62d06c81713b47b76330b2147a10386c