btm: Emit BTM.ShortVisitNeighbor to help distinguish multiple visits.

When the user hits the back button, the old UKM source id is used for
the new visit. This makes it difficult to evaluate multiple visits to
the same page. By adding a ShortVisitId metric and emitting events on
the pages before and after with that same visit id, we should gain
visibility.

Remove singular="True" for BTM.ShortVisit in ukm.xml, because we
sometimes want to emit another event when the user navigates back to the
same page.

Add some more details of special values to the TimeSinceLastInteraction
metric summary.

UKM collection review: go/btm-shortvisit-ukm

Bug: 400503246
Change-Id: I6c68a815ccdaa5197c24583684b13a307edc9c9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6329425
Reviewed-by: Andrew Liu <liu@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ryan Tarpine <rtarpine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429499}