Makes acknowledgeNewUserDisclaimer() a @TestApi

It's already a @SystemApi, but it's used by CTS so it needs to be
allowlisted as @TestApi, otherwise the test fails with a
NoSuchMethodException.

Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_newUserDisclaimer
Test: m update-api

Bug: 217753260

Change-Id: Ied5d86a5c067d57a4ae9bbc4d753481879d2e89e