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AmbientPhotoController and AmbientPhotoCache are only necessary
when a user is logged in and ambient mode is enabled. Construct them
at that time, and destruct them if ambient mode is disabled.

Schedule fetching backup images when AmbientPhotoController is
constructed, rather than in OnFirstSessionStarted.

BUG=b:176094707
TEST=ash_unittests --gtest_filter="AmbientControllerTest.*"

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: Id87bb16e6d4cc14074dd9bdb352fb5795d0f8915
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605744
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842133}