BMediaRoster: Reintroduce the undertaker class

* This has been necessary due to the undefined call order of
of static objects. Fixes #12315.
* The bug has been caused by the linker which free unused resources,
making the BMediaRoster to run in a zombie state. In this state
anything such as a message could make the looper to crash.
* The class is reintroduced with some differences though, we are
going to protect it from another thread calling Roster() while the
BMediaRoster is quitting and implement BMediaRosterEx::Quit.
* Unregister registrar notifications before we quit our thread. Avoid
to uninitialize anything from QuitRequested as it may cause problems.

headers/private/media/MediaRosterEx.h

@@ -37,6 +37,8 @@ public:
 							BMediaRosterEx(status_t* out_error);
 		virtual				~BMediaRosterEx();
 
+		virtual void		Quit();
+
 		status_t			SaveNodeConfiguration(BMediaNode* node);
 		status_t 			LoadNodeConfiguration(media_addon_id addonid,
 							int32 flavorid, BMessage* out_msg);