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This patch adds the Get/Update Settings APIs so that we can show and
update it in the Chrome OS Settings ambient mode page, which will be
implemented in a follow up cl.

Bug: b/148483925
Test: manual
Change-Id: Ia74d875813860a6abbfdc01479a67fed0c029fb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125685
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755531}