Private DNS: Do not apply user restriction to system

Do not apply the user restriction for changing the Private DNS settings
to the system caller.
That will fix a bug where the DevicePolicyManager API for setting the
values of Private DNS-related settings would not apply.

Bug: 120416121
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: I26618a91224ac06e23ab86e96bc9b59d5325d3a0