Initial implementation of the download manager public API.

The implementation is in android.net.DownloadManager, which is
obtained through Context.getSystemService().  Right now this class
acts as a simple wrapper on top of the existing DownloadProvider,
exposing a simple interface to a subset of DownloadProvider's
functionality.  There are several TODOs for features that require
changes to the underlying download manager implementation.

Change-Id: I2f26e51b60b6e82af8478ac7ccd895667df095b6