DownloadProvider: Add transfer speeds in notification

This will show transfer speed in notification when in expanded mode.
For multiple files, the combined speed are displayed.

Screenshot: http://i.imgur.com/xqLRvMo.png

PS1: Initial commit
PS2: clean code
     add missing declarations & import

Change-Id: I801dbe61c7ee59d0c1d14d5851ad6dc3a7678499