Remove averageTime check in DnsServerSelection test

The test is flaky because averageTime is sometimes over than 20ms.
Ensuring dns1Count > dns2Count > dns3Count should be basically enough
to ensure the feature's effectiveness.

Bug: 182085418
Bug: 180530890
Test: ResolverTest.DnsServerSelection passed
Change-Id: Ia18ee2dbe4d2f6db279ca869040017296a94f1ba