guest_os: Session tracker stays in sync through Chrome crashes

We need to track some state in order to handle home directory sharing in
the files app, but we lose all that state if Chrome crashes. Use the new
ListRunningContainers and GetGarconSessionInfo methods to get back in
sync after a crash, which allows Crostini containers (once
http://crrev/c/3801773 lands), to still share files after a Chrome
crash.

Bug: b:217469540
Test: Unit, deploy, crash Chrome and remount (with above CL)

Change-Id: I1ee8a55e0cf14ceac81dce1a2a18dd2e2e41c2cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3802295
Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/main@{#1031748}