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This change adds a new autotest API: isArcPackageListInitialRefreshed,
to check whether the ARC package list has been refreshed.

Bug: 982538
Test: Run the tast testcase added in
Change-Id: I1b9633be08098c9dcb23609fdc0332d09d1f545b
https: //chrome-internal-review.googlesource.com/c/chromeos/platform/tast-tests-private/+/1948653
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981249Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729710}