base

Support new URI structure for download provider.

I'm changing the URI structure for the download provider a bit.  The
download provider will now support two views into its data:

* content://downloads/my_downloads... will basically be the current
  view, which is limited to downloads owned by the calling UID
* content://downloads/all_downloads... will include all downloads in
  the system, but will require special permission

In addition to making things more clear, this change will allow for
granting URI permissions to individual downloads via the
/all_downloads/... URI.

This change includes the framework changes necessary to support the
new structure.  The bulk of the work will be in the DownloadProvider
code itself.

In addition, this change makes DownloadManager return a content URI
rather than a file URI for /cache downloads.  This avoids any
confusion in clients, which wouldn't be able to open the file
directly, and better supports granting permissions to viewers.

Change-Id: Ie548b927817ac774111990dd0c9d26aaf979d1ea