Make PDF OCR Always Actie pref not syncable.

PDF OCR Always Active prefs is not synced between different devices.
This is done to comply with Screen Reader prefs, and avoid fluctuations
on different devices if users just needs the service on some devices.

AX-Relnotes: n/a
Bug: 1443341
Change-Id: I9ad1007517cd2b4254756a8b13154d4f7da7f64c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4711733
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Dominic Battre <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1174929}