MediaStore: Make definitions for Files table public

This provides a public API for the new MediaProvider support for
arbitrary file types.  MediaProvider is no longer limited to supporting
only media (audio, video, image and playlist files).
This also allows querying across multiple media file types and supports
navigating the directory hierarchy of the media storage via database queries.

BUG: 2984284

Change-Id: I6222a6d601a4641cc7e544335e45d05b194532b1
Signed-off-by: Mike Lockwood <lockwood@google.com>