Retain DownloadManager Uri grants when clearing.

As part of fixing a recent security issue, DownloadManager now needs
to issue Uri permission grants for all downloads.  However, if an app
that requested a download is upgraded or otherwise force-stopped,
the required permission grants are removed.

We could tell DownloadManager about the app being stopped, but that
would be racy (due to background broadcast), and waking it up would
degrade system health.  Instead, as a special case we now only
consider clearing DownloadManager permission grants when app data
is being cleared.

Bug: 32172542, 30537115
Test: builds, boots, app upgrade doesn't clear grants
Change-Id: I7e3d4546fd12bfe5f81b9fb9857ece58d574a6b9