Improve notification UX.

- Add a proper notification icon (courtesy of Joey Rizzoli)
- Use separate notification modes to allow the user to dismiss the
  notification when music isn't playing.
- Always show notification regardless whether an activity is in
  foreground to ensure the notification doesn't pop up out of the blue
  when ending playback inside the app and leaving it afterwards.
- Keep notification timestamp constant when updating it

Change-Id: I2fa9a56ff31ab2874d6d96786e6b80695397702b