DPM.createAndManageUser should work even with DISALLOW_ADD_USER set

For this, the DPM calls a new function
UserManagerInternal.createUserEvenWhenDisallowed() instead of
UserManager.createUser(). This calls
UserManagerService.createUserInternalUnchecked().

Also, only the system user is allowed to call this method, otherwise
a security exception is thrown.

Bug: 26952210
Bug: 26786199
Change-Id: I69c16354898d68592d13f5f53b840551f7ad4779