Slice by state in DurationMetricProducer

- Added #onStateChanged logic to DurationMetricProducer
- Only SUM aggregation type supports state slicing.
OringDurationTrackers now track durations keyed by different state
values.
- Added unit tests for DurationMetricProducer state changes

Test: bit statsd_test:*
Bug: 145838038
Bug: 136566566
Change-Id: I735c7deee96639c32cad4b907e59b354dfd98d77