Bring DownloadManager API inline with AOSPA

Change-Id: I9468e59617e2e6fd01a622c1c499ee52a096d291
Download: add to support pause/resume download by manual

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.

Change-Id: I606b48ca20f43bcc6c119683818c2ab6ff03bfe5