Sync: Add active device timestamp field

Add the active device timestamp field to device info. This is
used in a followup CL to allow floating workspace to use
DeviceInfo to detect recent devices and auto signout older devices.

BUG=b:339897041
TEST=None

Change-Id: Id5f8e51231caf603a806cd45949205ef1c5a1c84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5545026
Reviewed-by: Victor Vianna <victorvianna@google.com>
Commit-Queue: Matthew Zhu <zhumatthew@google.com>
Cr-Commit-Position: refs/heads/main@{#1325643}