[Download]Add download_files table schema and create table sql statement

Signed-off-by: Ethan Shan <shan0xiao0xi@gmail.com>
[Download]Add COLUMN_MD5 column to download_files table

Signed-off-by: Ethan Shan <shan0xiao0xi@gmail.com>
[Download]Add manipulate download_file tables API to DownloadFileDataSource

Signed-off-by: Ethan Shan <shan0xiao0xi@gmail.com>