rework notifications to work with new UX

Based on our conversation from the last UX Overhaul meeting, here's our conclusions based on my memory:

• only one notification to represent all install related events (update available, update downloaded, install downloaded)
• clicking on the notification opens My Apps, e.g. #709 (comment 13739800)
• if the notification is currently representing only a single action, then there is an action button in the notification, e.g. "Install" or "Update"
• index updates only show a notification while they are in progress (ultimately, this will totally disappear), and it will be distinct from the update/install notification
• F-Droid notifications should not show on the lock screen at all

@crwinfrey @pserwylo @commonsguy @relan @NicoAlt