[1/2] Add to support pause/resume download

We add pause and resume API here so that application can
pause and resume downloads manually by DownloadManager.
This contains two points:
1. add pause and resume download APIs.
2. add another paused reason for this download status.

PS2/3:
* Fix code.

Change-Id: I7f01aa3890589272cd2e03d474771ed9619df2d3