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This change uses prefs to store and retrieve app ids for the apps
registered for updates.

Bug: 1020285
Change-Id: I7dea36049e6e09ff7039b61f79acb268e94baddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053584
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745125}