Write eventlog entries for notification visibility

Keep track of notifications shown in the notification panel
and log them via eventlog.

Added the following eventlog tag:
  * notification_visibility_changed

Bug: 13442201
Change-Id: Ifb74ec2492967dc787b4de86db7d7f00be6c7f74
diff --git a/services/core/java/com/android/server/EventLogTags.logtags b/services/core/java/com/android/server/EventLogTags.logtags
index 9768934..5083d44 100644
--- a/services/core/java/com/android/server/EventLogTags.logtags
+++ b/services/core/java/com/android/server/EventLogTags.logtags
@@ -63,6 +63,8 @@
 27500 notification_panel_revealed
 # when the notification panel is hidden
 27501 notification_panel_hidden
+# when notifications are newly displayed on screen, or disappear from screen
+27510 notification_visibility_changed (newlyVisibleKeys|3),(noLongerVisibleKeys|3)
 
 # ---------------------------
 # Watchdog.java