Create deviceowner gated APIs for creating and removing users in devicepolicymanager.

This will allow DMAgent to manage users for EDU's cart model user case.
Bug: 15015887

Change-Id: I1eadf1701cb75fc4b50eb1a0df1525eff818286e
(cherry picked from commit be9f43b9618ec25121b151d247eb0285fb6554b3)
4 files changed