Bug 1581577 - Fix ICE duration telemetry high value; r=bwc

The Telemetry::AccumulateTimeDelta function reports in milliseconds but I had
assumed seconds when I added the hostname obfuscation ICE duration histogram.
This change adjusts the "high" value accordingly.

With the telemetry we've collected to date, 60% of the values are below 600
milliseconds, so I think that setting a high value of 30 seconds should be
sufficient even if there is a long tail. Since this does not change the data
being collected, no additional data-review is required, but it does require
renaming the key.

Differential Revision: https://phabricator.services.mozilla.com/D46045

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extra : moz-landing-system : lando