Tracks unknown condition states.

When statsd starts or is killed, the conditions are not initialized.
Tracks how many buckets are skipped because the condition was unknown.

This change does not change the behavior of statsd. In the future, we
might want to invalid buckets with a condition that has been partially
unknown.

Bug: 124100912
Test: atest statsd_test
Change-Id: Ie3d2eb5e30c21dda3203ad30f9b486961ff570cf