Subsampling DNS event for mDNS

Currently, a device generates some mDNS queries when the user uses
mDNS .local resolution.
Using the query info by subsampling events based on how interesting
they are. Because the number of mDNS query is much less than DNS
query, the mDNS subsampling rate is higer than DNS query.
- if return_code == 0,2,7 -> log 1 in 1 event
- if return_code == default -> log 1 in 1 event
Also allow to use experiment flag to update sub-sampling denom.

Example for dumpsys dnsresolver:
  NetId: 100
    DnsEvent subsampling map for MDNS: default:1

Bug: 197092658
Test: cd packages/modules/DnsResolver && atest
      m statsd_testdrive and run "statsd_testdrive 116"
Change-Id: I76073aa9a1cea43bda2675334592ed22e96a238e