Subsampling DNS events to reduce logs

Currently, each device generates about 4000 queries per day.
Reduce the number of events by subsampling events based on
how interesting they are:
- if return_code == 0 -> log 1 in 100 events (random sampling)
- if return_code == EAI_NODATA -> log 1 in 10 events (random sampling)
- else -> log 100%
Also allow to use experiment flag to update sub-sampling denom.

Example for dumpsys dnsresolver:
  NetId: 100
    DnsEvent subsampling map: default:1 0:100 7:10

Bug: 119862317
Test: manual test with using setprop to change subsampling map
      atest

Merged-In: Ibbe3c653bdf45fc03b56a61e3401fad11c05a010
(cherry picked from commit 79e5185780a94db0b2356edf987239da459433f4)

Change-Id: I170dda9247e94a17a7cac9e7e414d592bb4dccbf