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Mock out all Ambient mode disk access in unit tests.

BUG=b:165481552,b:169813071
TEST=ash_unittests --gtest_filter="AmbientPhotoControllerTest.*"
TEST=ash_unittests --gtest_filter="AmbientPhotoCacheTest.*"

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: Ic063e6b7686a74c67fbe7a23e146a9e1640abfd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558590
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838748}