Add State to statsd config language

Introduce State, StateMapping, StateGroup, and MetricStateLink into the
statsd config language and add a slice_by_state field in CountMetrics
for singular and aggregate state slicing.

Bug: 136566566
Test: bit statsd_test:*
Change-Id: Icbb50f25d1c9f5bec0a2f07e6b977820ade289e6