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
diff --git a/core/java/android/app/DownloadManager.java b/core/java/android/app/DownloadManager.java
index 570e494..592a63b 100644
--- a/core/java/android/app/DownloadManager.java
+++ b/core/java/android/app/DownloadManager.java
@@ -272,6 +272,14 @@
      */
     public static final String EXTRA_DOWNLOAD_ID = "extra_download_id";
 
+    /**
+     * When clicks on multiple notifications are received, the following
+     * provides an array of download ids corresponding to the download notification that was
+     * clicked. It can be retrieved by the receiver of this
+     * Intent using {@link android.content.Intent#getLongArrayExtra(String)}.
+     */
+    public static final String EXTRA_NOTIFICATION_CLICK_DOWNLOAD_IDS = "extra_click_download_ids";
+
     // this array must contain all public columns
     private static final String[] COLUMNS = new String[] {
         COLUMN_ID,