Update NRTrackerExoPlayer to obtain isLive attribute from the player

[asllop]

The media3 BasePlayer supports isCurrentMediaItemLive(), which returns whether the current media item is live. We can update the NRTrackerExoPlayer to get this attribute OOTB:

    @Override
    public Boolean getIsLive() {
        return player != null && player.isCurrentMediaItemLive();
    }