Make settings backup/restore work in the new multi-user world

1) Properly handle restores of settings elements that have been migrated
   to the new global namespace

1) Back up and restore the new global settings namespace

3) Make sure to back up / restore the global entity
   ENABLE_ACCESSIBILITY_GLOBAL_GESTURE_ENABLED

Bug 7249405

Change-Id: Ibfa9930ea4d0e16c7635697e8c631b155e4c0cb2