DownloadManager: add to pause/resume download by user

We add pause and resume APIs here to make application can
pause and resume downloads by DownloadManager.

Code change contains:
  - add pause and resume download APIs.
  - add another paused status for this case.

Change-Id: I5bf4f15b2464009595f7926522ca6d5f1b0e2b8d

Conflicts:
	core/java/android/app/DownloadManager.java