Private DNS API: Follow-up on review

To follow up on the previous review, rename the success constant from
"success" to "no error" since the enum it's associated with now refers
to error codes.

Bug: 124301971
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: I6b82dd44cbf2c18fb8c1673a272f22f0e8b39dbb