Fix ValueMetricProducer_test on hwasan

TestUseZeroDefaultBaseWithPullFailures was failing on hwasan builds
because the test had a reference to an Interval object that was
deleted. The test tried to get another Interval object and assign it to
the reference. However, what this really did is change the Interval
object that the reference referred to. Since that Interval object had
been deleted, hwasan failed the test.

This fixes the issue. Also clean up some usages of auto to make
the test more readable.

Test: bit statsd_test:* on blueline_hwasan
Bug: 152354576
Change-Id: I392740770715391c727d190b1ed8ba22cc01210f