Heads up master switch (1/2)

Create a global heads up switch to enable/disable the same.
The user still need to set each app in Settings -> Apps

PS: disabling through master switch will automatically turn of  call ui and mms heads up notifications

Change-Id: I650e230d275247251435c5cfeb728e375e927045
[1/2] Base: DownloadManager: add support pause/resume/stop notifications

Add pause, resume and stop functions to make user can pause, resume and stop downloads by manual in downloads application.

Change-Id: I82ffff5a539448faab8933e2b47591199bf24cb9
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

Conflicts:
	core/java/android/provider/Downloads.java