Add metrics to track for voice interaction events per intent target.

We currently only track the differences per-source. Splitting these by
target will allow us to see the differences between system and
Assistant behavior.

This also syncs the end dates for these histograms to 8/22.

(cherry picked from commit c73e2fd)

Bug: 1184843
Change-Id: Ia895b82d20d083434fef25491654a619a22d98bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2737777
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Brandon Wylie <wylieb@chromium.org>
Commit-Queue: Josh Simmons <jds@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#860828}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2747811
Cr-Commit-Position: refs/branch-heads/4430@{#326}
Cr-Branched-From: e5ce7dc-refs/heads/master@{#857950}