[chromebox_for_meetings] Add DB scaffolding

Add the skeleton for flushing data to persistent storage.
A future commit will introduce the child class that contains
the real backing for the DB. The current implementation does
nothing, for now.

This DB will be used for LogSource recovery in the event of
a crash or reboot, to reduce duplication in uploaded logs.
The goal is to continue from the last-read file offset, rather
than starting at the beginning every time.

Bug: b:322505142
Change-Id: I20ea03431723a52cb50266e99ebed5f727f49871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5515657
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Niko Tsirakis <ntsirakis@google.com>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1301303}