Prevent incorrect alert triggering

Alerts were being incorrectly triggered in pulled value metrics because
a diffed value is always assumed to be over one bucket interval, which
may not be true when multiple buckets are skipped.

Bug: 159390273
Test: atest statsd_test
Change-Id: Iada3df54ebb30353572a14e0db86bed49c5515b0
Merged-In: Iada3df54ebb30353572a14e0db86bed49c5515b0
(cherry picked from commit ec72e875)