Add getDnsNetId cmd in DnsProxyListener

This command is used to get the netId which resolver
will use for sending DNS query.

Correst ResponseCode.h from const to constexpr
for test.

Input:
    cmd format: getdnsnetid netId
               -netid: selected netid of the process

Output:
    Success: 1. ResponseCode::DnsProxyQueryResult
             2. dnsNetId

    Fail:    1. ResponseCode::DnsProxyQueryResult
             2. -errno

    Possible errno:
                   EINVAL
Bug: 129530368
Test: built, flashed, booted
          system/netd/tests/runtests.sh

Change-Id: Iaeabb9330a963b30da5948494432118893e084ac