Ash: Prevent informed restore screenshot in multi-user scenarios

This change prevents the screenshot from being taken under two
multi-user conditions:
1. A secondary user is active at the time of sign-out.
2. The primary user is active, but a visible window from another user is
   present on the current desk.

New UMA metrics are added to track when the screenshot is skipped for
these reasons.

Bug: b:434850930
Change-Id: I6f4c2ca34926921047713f32ff9b75efd716b195
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6839958
Commit-Queue: Jun Ishiguro <junis@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503146}