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The CL add the AwVariationsSeedFetchService which is a JobService to fetch Finch
seed data from the Finch Server. It reuse some of the code from the
VariationsSeedFetcher, so I refactory the class to expose some reusable
function. The download is triggered by AwVariationsConfigurationService which is
a bound Service to managing the Finch seed data system-wide. The fetched seed
data is going to store in the local directory belongs to the Android WebView.
This is just a prototype of the Variations Seed Fetch Service which is one part
of the work of adding Finch to Android WebView, so there will be another CL
related to the AwVariationsConfigurationService.

BUG=733857

Review-Url: https://codereview.chromium.org/2975693002
Cr-Commit-Position: refs/heads/master@{#488719}