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This is the first step to migrate the Assistant related prefs.
Currently they are defined in the following places:
https://cs.chromium.org/chromium/src/components/arc/arc_prefs.h
https://cs.chromium.org/chromium/src/chrome/browser/ui/ash/assistant/assistant_pref_util.h

It migrates the AssistantConsentStatus preference. The other
Assistant related prefs will be migrated in follow up cls.

- Add AssistantPrefs, register kAssistantConsentStatus as public
preference in browser, and as foreign preference in ash and Assistant
service.
- Remove old logics and mojom methods related to kAssistantConsentStatus
in browser/ash
- Add AssistantPrefsController in ash, also add an observer class.
- Add AssistantPrefsControllerTest in ash_unittests.

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

Bug: b/110211045
Test: Run AssistantPrefsControllerTest.* in ash_unittests
Change-Id: Id9a1be119174258f53b08f96208cc4c0c8a03133
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1694403
Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678058}