Clean up notifications.

This patch:
- uses NotificationCompat.Builder for building download notifications
- removes the custom RemoteView for the notification in favor of native
  notifications (which are simpler and look better on all versions).
- fallback to x/100 in the subtitle for pre-ICS devices that can't show
  progress bars in notifications.
- improves notification text (show sura name without ayat if downloading
  gapless, show canceled instead of failed when download was canceled).
- fixes notification buttons being broken due to recent changes.