Returns the active desk index for unassignedworkspace.

Problem: In lacros, when toggling "All desks" in menu,
the desk menu has to be assigned back to the active desk.
But it still stays "All desks".

This is because we clear kWindowWorkspaceKey to UnassignedWorkpace
if "All desk" is toggling.
To handle this, we have to return the active desk index
for unassignedworkspace value.

Bug: 1260803
Change-Id: I2f8c3037ce3f05f3672736d5a43bf08e34af5b4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3226197
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Minju Kim <mkim@igalia.com>
Cr-Commit-Position: refs/heads/main@{#932872}