Various UI and UX fixes.

- Don't show notification if no updates are available.
- Only notify of new updates (that is, newer than installed) instead of
  notifying of everything available
- On download finish, show a notification instead of directly launching
  the activity and disrupting the user's workflow; allow installing the
  update from that notification
- Handle cancel correctly (abort any pending HTTP requests)
- Some cleanup

Change-Id: I86411d11516fa1cca58a357d8feeb790d14bbbc6