Log notification clicks

Emit notification_clicked log events when a notification
is clicked from SystemUI.

Also refactor the onNotificationClicked method to work with
a key instead of individual notification params.

Change-Id: Iffd15e95d46371b2ae7bfd00b2c348d9f4cf5d14
diff --git a/services/core/java/com/android/server/EventLogTags.logtags b/services/core/java/com/android/server/EventLogTags.logtags
index 5083d44..6fab37c2 100644
--- a/services/core/java/com/android/server/EventLogTags.logtags
+++ b/services/core/java/com/android/server/EventLogTags.logtags
@@ -65,6 +65,8 @@
 27501 notification_panel_hidden
 # when notifications are newly displayed on screen, or disappear from screen
 27510 notification_visibility_changed (newlyVisibleKeys|3),(noLongerVisibleKeys|3)
+# when a notification has been clicked
+27520 notification_clicked (key|3)
 
 # ---------------------------
 # Watchdog.java