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This CL adds the preferred apps storage in the App Service Server,
and implement methods to sync the change between App Service Server
and the Subscribers.

BUG=853604

Change-Id: Iba78cd0d02d95464de038c7d97f267d75bdcec9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866228
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708526}