Add support for KEYCODE_NOTIFICATION.

Let KEYCODE_NOTIFICATION toggle the notification panel handler defined
by config_notificationHandlerPackage.
Add log messages if config_notificationHandlerPackage is not defined and API is called.

Bug: 158661029
Test: manually tested on ATV device with adapted launcher
Change-Id: Id99f8922785f41e5acb23f1b45386a59710aa810