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: 119862317
Test: ./out/host/linux-x86/bin/statsd_testdrive 116

Merged-In: Iab542b32a8a18ae0bdbd8e005c430a876f056806
Merged-In: I9d37cac0bca53b3d7c6751ca69c1d304dfe17098
(cherry picked from commit 60d4f1a5)

Change-Id: If380a730942195db91a66944f103674c539ca7ad