[EoS] Fix background task scheduling across updates.

This is a one-off solution just for Explore Sites. It does the following:
* Changes the job ID for ExploreSitesBackgroundTask 100->101
* Deprecates job ID 100
* Cancels any tasks with job ID 100 when they are executed by JobScheduler
* Forces ProGuard to keep the ExploreSitesBackgroundTask class name
  without obfuscation.

Bug: 908476
Change-Id: Ib769e6678a82771a68dec86c04a73621f21fa9e1
Reviewed-on: https://chromium-review.googlesource.com/c/1370318
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Dmitry Titov <dimich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615657}
7 files changed