Prevent screenshot when admin disable screenshots for all users

* An IT admin has the ability to disable screenshots
  for all users when the device has a DO or COPE PO
* When screenshots are disabled for all users, prevent
  a screenshot from being taken instead of taking a
  black screenshot

Bug: 217558483
Test: Manual testing with TestDPC
Change-Id: I167996bb337cc906b5475089f674fd9c37dff808