[Omnibox] Record metrics for answer actions

This adds shown/used histograms, bucketed by the answer type.

Bug: 326368632
Change-Id: Ic123903526985dbf6fddf0204a967dfa27a973b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617835


Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Reviewed-by: Jennifer Serrano <jennserrano@google.com>
Cr-Commit-Position: refs/heads/main@{#1313609}