Have DownloadManager pass file URIs for legacy external downloads

DownloadManager was set up to return file URIs for public API
downloads to external, but to return content URIs for all others.
This included browser downloads, which are on external but don't go
through the public API.  Since the new downloads UI uses
DownloadManager, it was affected by this change when the user opened a
file from the UI.  Apparently numerous things don't handle content
URIs well, including music preview (clicking on downloaded mp3) and
package installer (clicking on downloaded APK), so this change makes
it generate file URIs for legacy external downloads.

Change-Id: I5c34d8ba188cc8b0d11865262cb451208be78eae