There are intent extra codes AppWidgetManager.EXTRA_CUSTOM_EXTRAS and AppWidgetManager.EXTRA_CUSTOM_INFO. What are they for? The docs state that it's "An intent extra to pass to the AppWidget picker", but who and when does the passing? If I understand right, the picker is invoked by the user, not by my code.
"An intent extra to pass to the AppWidget picker" indicates that an Android process that can manage widget picking will receive this intent. The process that can use the picker is an AppWidgetHost, like the Home screen. When the user invokes the picker, it is the Home screen that is processing the intent and hence using this extras.