Allow notifications to not specify a contentIntent.

If they don't, the click events will be passed through to the individual
views in the notification view, which may have their own PendingIntents
attached.

Previously, it was against the UX spec to allow this, but now we are
changing that and will have buttons in there.

Change-Id: Ie3b2e96c6a1c4449fa86ed571f3ad0f047320d31

services/java/com/android/server/NotificationManagerService.java

@@ -741,10 +741,6 @@ public class NotificationManagerService extends INotificationManager.Stub
                 throw new IllegalArgumentException("contentView required: pkg=" + pkg
                         + " id=" + id + " notification=" + notification);
             }
-            if (notification.contentIntent == null) {
-                throw new IllegalArgumentException("contentIntent required: pkg=" + pkg
-                        + " id=" + id + " notification=" + notification);
-            }
         }
 
         synchronized (mNotificationList) {