[FSA] Propagate former_relative_path for change events with kMoved type

With crrev.com/c/5408211, `ChangeInfo.moved_from_path` is reported on
move events. This CL passes this path info as former_relative_path to
the renderer-side, which already reports it as `relativePathMovedFrom`
to the web API.

Change-Id: I19b3de225ec94983f31e4ea2ab34bf926d76e275
Bug: 321980205, 341135410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5529749


Reviewed-by: Nathan Memmott <memmott@chromium.org>
Commit-Queue: Daseul Lee <dslee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1310107}