Slice by state in ValueMetricProducer

- Added #onStateChanged logic to ValueMetricProducer
- #onDumpReport fills in slice_by_state values in ValueMetricProducer
- Base information is tracked in a separate data structure and keyed by
dimensions_in_what HashableDimensionKey. The current state key is also
stored in base info so we can close the interval for this state once we
get a new state key.
- Added unit tests for ValueMetricProducer onStateChange

Other changes:
- mCondition parameter was removed from #pullAndMatchEventsLocked and
 #accumulateEvents because it is unused in these functions.
- The event key is initialized in each metric's internal
 onMatchedLogEvent function. This allows ValueMetric to skip events that
 have primary keys that do not match the current state change primary
 key.

Test: bit statsd_test:*
Change-Id: I565c6d251262be57abf10fdef243b8bdc01f3772