Check the latest documentation.
This feature seems to be called Screen pinning now, and the App does not have to be the device owner. If the device owner is not set the user will be prompted to accept.
There are two ways to activate screen pinning:
Manually: Users can enable screen pinning in Settings > Security > Screen Pinning, and select the tasks they want to pin by touching the green pin icon in the recents screen.
Programmatically: To activate screen pinning programmatically, call startLockTask() from your app. If the requesting app is not a device owner, the user is prompted for confirmation. A device owner app can call the setLockTaskPackages() method to enable apps to be pinnable without the user confirmation step.