Hidden API to control sorting of download manager queries.

This change adds a hidden API to sort download manager queries by size
rather than by time last modified.  This is necessary for the system
download manager UI but, in the interest of keeping the new public API
as minimal as possible for now, I'm leaving @hide on this method.

This change also changes the DownloadManager.COLUMN_ID column to be
identical to BaseColumns._ID, since some code relies on this (such as
CursorAdapter).

Change-Id: Ib89b614d63e988974d4486b122b39365b35b5269