[2/2] Add to support pause/resume download

User can manually pause current download and resume to continue download remaining data of the file.
add a progressbar to show download progress. (only legacy View)
add pause/resume/stop/retry into notifications
add remaining time into notifications

PSs: Code cleanup.

Change-Id: I7c2c6043bd6e26c4d4b0c74265f87629967302a0