Anomaly detection is per dimension

Anomalies are now detected and declared per dimension. This means that
declareAnomaly now gets access to the key that is responsible for the
anomaly. Moreover, the refractory period is per dimension, not overall.
So a second anomaly for the same metric but a different dimension can
fire within the first dimension's refractory period. Thus, if app A
misbehaves and app B misbehaves shortly thereafter, they will both be
detected.

Eventually this key will be passed to the subscribers, although this cl
doesn't do anything with it.

Test: adb shell data/nativetest64/statsd_test/statsd_test
Change-Id: Id76856dc44fe9ecf91ac81a423e84f97c81d30ab