Added a DevicePolicyManager.setDeviceOwnerOnly() method.

It will be used by the test infra to just set the device owner,
without setting the profile owner on current user on systems
using headless system user.

Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testDeviceOwnerSetup
Test: adb shell dpm set-device-owner --user 0 --device-owner-only com.afwsamples.testdpc/.DeviceAdminReceiver
Test: m update-api

Fixes: 201313785
Bug: 192395248

Change-Id: I41c2cf187c1eceebe7c6ef30f9cff693b9dcbc15