[2/2] Pause and 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.

[2/2] 暂停和继续下载

添加暂停和继续的API从而让应用可以通过DownloadManager
来手动暂停和继续下载.

这包含两项:
1. 添加暂停和继续下载的API.
2. 添加一个新的暂停下载的状态.

[1/2] in platform_frameworks_base