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This CL adds SmartDimComponentInstallerPolicy which takes care of
downloading, installing smart dim components and feeding them into
SmartDimMlAgent to update its download_worker_.

It also takes care of the smart dim experiments, concretely, it
fetches and uses exact expected_version from the server, while
expected_version can be configured by a finch flag. Therefore we
can use finch to control the version it uses.

expected_version from the server, no matter what other versions
already exist in the disk.

Bug: 1018065
Test: Tested on DUT, it successfully fetches and uses the
Change-Id: I2759d55291e9424c255b977ccba2f1632dc44f43
Cq-Depend: chromium:1903168
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1990885
Commit-Queue: Xinglong Luan <alanlxl@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751810}