[Previews] Generalizes Previews.Triggered UMA to any preview type [2]

Change 1385324 turned out to be an incomplete attempt to capture the
triggered ECT value for each preview type because it was in a scope
that is not called for every preview type. Here, we move up the uma
logging to a higher level that does have visibility for each type.
Also, renaming the histogram slightly (appended "2") to differentiate
what is logged.

Bug: 916270
Change-Id: I52aad5bec8ac5607ffc7cdce8849054363fe16ab
Reviewed-on: https://chromium-review.googlesource.com/c/1399705
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621234}
5 files changed