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. https://gitlab.com/fdroid/fdroidclient/issues/709#note_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