Fix statsd crash due to bad bucket index

Statsd crashes because predictAnomalyTimestamp requests past buckets
that occurred before time began. That's fine, but statsd needs to know
that the data before time began was 0 (instead of reading before the
beginning of the array and crashing).

AnomalyTracker's use of bucketNumbers is in general risky, and should be
auditted. But this cl will fix the current crashing.

Bug: 73825954
Test: Definitely necessary. Will write during audit.
Change-Id: I990ff134153f290d3089bfe3440d838f47996b63