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
ExternalStorageProvider: use mount point in device list

For me its is extremely unlogical that only the volume label is used
in the device list. Even more because it can be an empty string
and for a normal android user this information is not visible
anywhere else.

Using the mount point and if available the device label
separated by "-" makes much more sense and shows the same
information as all other apps that access external storage

Change-Id: I3fc3675af6640e37aa999b03ae2de2858237c638