Adds anomaly detection to max duration metrics.

Statsd supports anomaly detection for max duration, which is
conceptually like asking for an anomaly on the current duration.
There was a bug previously in the logic for max duration since we
recorded a duration every time a dimension went into pause. Now, we
only record a max duration when the dimension goes into stop.

When a dimension inside the MaxDurationTracker leaves the started
state, we need to check if we still need to keep an anomaly alarm
set. It's possible that we just immediately set an alarm with the
same timestamp as what was just deleted.

Test: Added some unit-tests and tested on marlin-eng.
Change-Id: I09c82cd266b4cc6e1c893ad166e602bf08baca0e