Report private DNS protocols in onPrivateDnsValidationEvent

PrivateDnsDohTest.QueryFailover fails in waiting for the second
onPrivateDnsValidationEvent if DoT validation result and DoH
validation result are reported at the same time.

To fix the flaky test, this change makes the validation results
distinguishable by private DNS protocols.

Bug: 198111329
Test: PrivateDnsDohTest.QueryFailover 200 times passed on cuttlefish
Test: PrivateDnsDohTest.ValidationFail 500 times passed on cuttlefish
Change-Id: I8649b21bff176c6f2a1158f04812c8c8e96ad7d9