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This is a reland of 84408458

Original change's description:
> ambient: Use backdrop APIs
> 
> This cl adds a framework to use backdrop APIs to fetch photos etc for
> Ambient mode.
> 
> Bug: b/148483925
> Test: manual
> Change-Id: I777b9269148c5317a18e763e911e4893e7ecbe2b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026258
> 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@{#740069}

Bug: b/148483925
Change-Id: I6a6b6d25feeb181e00a207d5e58518533a8e263c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053038
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742823}