Small fix to anomaly tracker in statsd.

If the config specifies only one past bucket for anomaly detection,
statsd doesn't want to store any past buckets since only the current
bucket being tracked in the Metrics Producer is used for deciding
if we hit the anomaly.

Test: Checked that statsd_test passes.
Change-Id: I7ca65bf7d2dfcb2d5c7d5c90f63f4a1c70fbc792