BMediaRoster: Fix initialization and destruction

* Improve consistency by adding a BMediaRosterEx destructor
  and using it for the specular functionality of ctor instead
  to use the father's class destructor.
* Avoid double initialization of MediaInitializer that
  becomes MediaRosterUndertaker.
* Remove superfluos call to BMediaRoster::Quit()
  in media_addon_server.

headers/private/media/MediaRosterEx.h

@@ -35,6 +35,7 @@ class BMediaRosterEx : public BMediaRoster
 {
 public:
 							BMediaRosterEx(status_t* out_error);
+		virtual				~BMediaRosterEx();
 
 		status_t			SaveNodeConfiguration(BMediaNode* node);
 		status_t 			LoadNodeConfiguration(media_addon_id addonid,