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

Merged-In: Ie81751b762a780ed843bea255c1507ed03660c0e
(cherry picked from commit 4bd0e67a)

Change-Id: I18c4c04ab4547dc29cf97abc49ad01563e534e26