birch: Improve subtitle for recent tab chips

The subtitle now appears like one of these:
< 1 hour ago * From Chromebook
1 hour ago * From Chromebook
2 hours ago * From Chromebook
Yesterday * From Chromebook

Bug: b:329358094
Test: ash_unittests --gtest_filter=BirchItemTest.*
Change-Id: Ibda709344be29f65226d6d9f4ed6a0286995540f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420652


Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282613}