Anomaly Alert declarations in StatsdStats

StatsdStats now tracks the number of times an anomaly is detected (per
config, per alert name).

Also adds a configKey to AnomalyTracker, which is needed, not only for
statsdstats, but also (in the future) for reporting the header
information to incidentd.

Bug: 67978682
Test: adb shell data/nativetest64/statsd_test/statsd_test
Change-Id: Ib254db7e1edb4f0f193f4772d17f14934cdf7e30