Fix flaky test PrivateDnsDohTest.PreferIpv6

doh_ipv6.startServer() fails at binding the address if the address
hasn't been released. Change the test to bind the address once only.

Bug: 198111329
Test: PrivateDnsDohTest.ValidationFail 500 times passed on cuttlefish
Change-Id: I6b347528927ec3afc7eddfa31784c1f34e4e7661