Fix dot_xport_unusable_threshold to work

Before this change, if dot_revalidation_threshold is unset, setting
dot_xport_unusable_threshold doesn't make the DnsResolver skip
unusable DoT servers. This change fixes the problem.

This change is a preparation before we set dot_revalidation_threshold
and dot_xport_unusable_threshold to some value.

Some tests can't pass if the two flags are set to 1, so temporarily
set the two flags in those tests.

Bug: 79727473
Test: set {dot_xport_unusable_threshold=1,dot_xport_unusable_threshold=1} and ran atest
      set {dot_xport_unusable_threshold=-1,dot_xport_unusable_threshold=-1} and ran atest
Change-Id: I262eb889716001f8cd41ed7b94be3bca7941862f