Audio switch management update

Created by: nicosang

I used the sample http://wowzaec2demo.streamlock.net/vod/definst/ElephantsDream/smil:ElephantsDream.smil/manifest_mvnumber.mpd in order to test audio switch management. I found the first downloaded init audio segment was always used for each language switch. With this specific sample, it works maybe because the two audio configurations are very similar. What I propose in this PR is to store init segment by adding the id of adaptationSet. Indeed, two audio languages could have the same quality in two different adaptationSet.