Settings: Let users disable the left-corner Keyguard widget

The application widget is a generic icon displayed on the lock screen
that allows a user to launch an activity.  This is part of the Appcast
flow.

Note: The intents necessary to manage this widget is protected by a
system|signature permission.

Change-Id: Ida245b8c096a24ca3d8bb9a09c6af383bf3295cf
Reviewed-on: https://gerrit.nextbit.com:8443/8255
Reviewed-by: chub <chub+gerrit@nextbit.com>
Tested-by: chub <chub+gerrit@nextbit.com>