birch: Only show "Join" button on chip for meetings that are in-progess

Calendar events with a video conference URL have a "secondary action"
that allows the user to click a "Join" button and join the meeting.
Only show the "Join" button if the meeting is in-progress or about to
start.

Bug: b:332964468
Test: ash_unittests --gtest_filter=BirchItemTest.*
Change-Id: Ic747a5f4d32dcd329e4ec8f9abdef253bcc05633
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5427172


Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283416}