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1. if an application designates a downloaded file to be mediascanner
scannable or not, store that fact in database.
2. use the above to determine whether a file shoudl be mediascanned or not
in DownloadService
3. implement code to return mimetype for the new Uri "/public_downloads"
introduced in CL: I1f5dd734e394db0056579a3a0c26862fee27981e

Change-Id: I5c062ad6d1b58306044cee49ff3827e908d27fd9