DownloadProvider: Add Download speed in notification

This commit adds the ability to show the current total download
speed in the expanded mode. This is applicable to both single
and multi file downloads.

Screenshots:
Multi  - http://i.imgur.com/3t7yAy7.png
Single - http://i.imgur.com/MMqmA22.png

Slim'ed by mnazim

PS
- move strings to slim_strings

Change-Id: I801dbe61c7ee59d0c1d14d5851ad6dc3a7678499