Use "all_downloads" instead of "my_downloads".

We can no longer return the "my_downloads" paths: if those Uris were
shared beyond the app that requested the download, access would be
denied.  Instead, we need to switch to using "all_downloads" Uris so
that permission grants can be issued to third-party viewer apps.

Since an app requesting a download doesn't normally have permission
to "all_downloads" paths, DownloadProvider now issues narrow grants
toward the owner of each download, both at device boot and when new
downloads are started.

Bug: 30537115, 30945409
Change-Id: I533125b36444877f54373d88922f2acc777e250b