Populate updated_hardware_class in system profile

Retrieve updated hardware class by calling the `runtime_hwid_tool get`
command in StatisticsProvider. Since the value can be updated about one
minute after the device boots, a file watcher is set up in
StatisticsProvider to monitor the updated hardware class file to keep
the value up to date.

Also, update ChromeOSSystemProfileProvider to get updated hardware class
from StatisticsProvider, and use it to populate the
updated_hardware_class field in system profile.

Bug: b:457942761

Change-Id: I3e92845271f0fe72fde23b537907714e7e262967
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7366611
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Allen Shih <allenshihmc@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1570141}