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Created StatusUploader class to manage the uploading of device status.

Refactored status uploading functionality in cloud policy infrastructure - the
timing and contents of the device upload are now controlled by StatusUploader,
while CloudPolicyClient exposes an API to create the upload job.

BUG=430908

Review URL: https://codereview.chromium.org/845313008

Cr-Commit-Position: refs/heads/master@{#313535}