Fix using EDNS0 when no private DNS validated

Do not add NET_CONTEXT_FLAG_USE_EDNS to android_net_context flag
if there is no private DNS server validated.

A server not supporting EDNS might respond with FORMERR or ignore
the request. In former case, we will remove OPT RR and retry again.
In later case, we will not retry, so the DNS query might timeout.

Also fix the bug which dns_responder responds to the query even
though it's been set unresponsive.

Bug: 120257033
Test: system/netd/tests/runtests.sh passed
      Checked packets not containging OPT RR when using cleartext DNS
Change-Id: I8250a800ddade0ff810445bc912ea5799b99ec8c