Keep shared downloads when apps are uninstalled.

When an app downloads files to external storage, keep those downloads
around for the user to enjoy after the app is uninstalled.

We still end up deleting files stored in internal cache directories,
and under package-specific directories on external storage.

Test: builds, boots, downloads on external storage remain
Bug: 30868200
Change-Id: Ib70f42aa764a8252fe67c6fba9d60b3350f5d5a4
3 files changed