Add one field(sampling_rate_denom) in NetworkDnsEventReported for recording sample rate.

The sample rate of DNS stats(to statsd) is 1/sampling_rate_denom.

Bug: 133111380
Test: ./out/host/linux-x86/bin/statsd_testdrive 116
Change-Id: I85acc8cb8ec4c38e95f7e97b0dd8da1294d9ceb3