Update the setting to use Global instead of secure, deprecate old one

Deprecates the Secure setting so it's clear we're not using it anymore.

Since this work is for a QPR we need to listen for both the new Global
setting as well as the old Secure setting so that CTS may pass without
code changes:
* When the secure setting is changed (only CTS uses it now), the global
  setting will also be updated.
* When the global setting is changed (i.e. switching developer toggle), the
  global setting is updated and so is the secure setting.

Test: atest NotificationManagerTest
Bug: 137876221
Change-Id: Ib4623689aa4ec1fc79e2ec130702fdf9785834d8