notification: add support for listing notifications

Apps displaying notifications might want to cancel existing notifications in some cases. For instance, when a user of a messaging app reads a conversation, the app should cancel all notifications about the conversation.

This works fine with the existing API when the app keeps running. But as soon as the app is closed, and restarted, the app has no way to cancel previously opened notifications because it doesn't know which are still opened and which notification IDs are used.

Android fixes this by allowing apps to list active notifications, and allowing apps to attach a "tag" to notifications (e.g. storing the conversation ID).