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This cl migrates kAssistantDisabledByPolicy preference from
assistant_pref_util.h to assistant_prefs.h

Also, removes the file assistant_pref_util.h and updates related tests.

Detailed design doc: https://goto.google.com/assistant-prefs

Bug: b/110211045
Test: Run existing tests
Change-Id: Id41932cc6aeb9dc9ac5926dd764ca8c468412341
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1705109
Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678578}