birch: Improve subtitle for calendar event file attachments

It will be one of:
* From upcoming calendar event
* From event happening now

Bug: b:329358094
Test: ash_unittests --gtest_filter=BirchItemTest.*
Change-Id: I9d62edd7c6427baae91dd45fe4dca6572a7953ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425694


Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282721}