Pull class AppMetaMimeCreator out of CreateAppMetaMimeThread

Instead of using a BMimeType to set the MIME type properties, it
operates directly on the Database object, though.

headers/os/storage/MimeType.h

@@ -158,7 +158,6 @@ class BMimeType	{
 			// for testing only
 
 		friend class BAppFileInfo;
-		friend class BPrivate::Storage::Mime::CreateAppMetaMimeThread;
 
 		virtual void _ReservedMimeType1();
 		virtual void _ReservedMimeType2();