add NOTIFICATION_CLICK_ constants to DownloadManager

DownloadReceive in packages/providers/Downloadproviders sends intents
when notification messages are clicked on.
When using public API of DM, added code to include download entry ids
to the above intents. The constants added here support that code.
intents when

Change-Id: Ibe53ccd9934c73175459e42e3d417eee69ae6735