AnomalyMonitor can get alarms based on timestamp

When StatsCompanion informs statsd that an anomaly alarm has fired,
statsd will need to check to see whether an anomaly has indeed occurred.
This will require determining which anomaly alarms have times in the
past (since that indicates that the anomaly did indeed occur). This
capability is now added.

Test: adb shell data/nativetest64/statsd_test/statsd_test
Change-Id: I5464c94634df70832d2723d8c4718277776d58aa