[1/2] 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.

[1/2] 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.

[2/2] in platform_packages_providers_DownloadProvider
AOSPA Pie

AOSPA Pie is original implemented by AOSPA team as a
replacement for AOSP navigation bar.
It shows the three floating navigation buttons (in
an arc) in immersive mode along with basic system status.

AOSPA 派

AOSPA 派由AOSPA团队原创, 用来替代AOSP导航栏.
他可在沉浸模式中(以弧形)显示三个悬浮的导航键, 以及
一些基本的系统信息.