private String getKey(AdaptationSet adaptationSet, int segmentNr) {
        return adaptationSet.group + "-" + segmentNr;
    }
Returns a unique key for a segment of an adaptation set. Just using the segment number as
key does not suffice because multiple adaptation sets (e.g. video and audio) have overlapping
segment numbers.
MediaPlayer-DASH/src/main/java/net/protyposis/android/mediaplayer/dash/SegmentDownloader.java