Cancelling a download cancelled the worker and marked the row CANCELLED but
never abandoned a PackageInstaller session that had already been staged for
install, leaking it. Add IInstaller.cancelInstall (no-op default, implemented by
SessionInstaller) and call it from DownloadHelper.cancelDownload.
Cross-process session persistence/reconciliation (committing a session staged
before a restart) is left as a follow-up; the startup session cleanup remains,
and is now cheap to recover from since CacheWorker keeps the downloaded files.
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