Fix flaky ResolverTest.DnsServerSelection due to flag reset

The test takes around 5 seconds. If the flag is reset in the middle
of the test, the test will be broken. This change ensures the flag
is set right before the test network is created.

Bug: 177695274
Test: ResolverTest.DnsServerSelection passed
Change-Id: I69ec2edd2b43be3cc4cc535c0bc0a589838e65c8