[Feature Request] Don't use polling for updating seekbar

[nic0lette]

Currently, NowPlayingFragmentViewModel uses a handler to cause updates to the seekbar and text.

It would be better to listen to the metadata + control callbacks and set an animator on the bar that calculates (duration of song - current position) / playback speed and animates the progress from current to 100% over that duration.

The text could be updated in the seekbar's update callback, which would have the added benefit of the text updating as the position is changed by dragging it.