bug:3069735 in Download UI app, handle deletes correctly

gingerbread.
(for more detailed comments, please see the dependent CL
Change-Id: Ib90eb9e647f543312c865d3bbf9a06fb867a648b )

2 changes in this CL.
1. add 2 new columns to downloads table in downloads.db:
    mediaprovider_uri = downloaded file's content_uri in mediaprovider db
    deleted = flag is set to true if a file is to be deleted
2. download UI app shows only those files whose 'deleted' flag is not set

Change-Id: I47c5f72b0a0b354fac3abe3a97752130b8189794