Fix flaky test PrivateDnsDohTest.ExcessDnsRequests

The test is flaky becuase the doh flag in DeviceConfig is
sometimes reset by someone else during the test. To fix
that, we have to make sure that the flag is set to 1 whenever
a network is created or destroyed.

This change also has some minor fixes.

Bug: 233024050
Test: atest
Change-Id: If4d10c3f37b120dea367af9362dde3d90cc7309c