Only set IsAvc for video streams

Also enables nullable for MediaStreamInfo
Makes more properties nullable that aren't always present

MediaBrowser.Model/Entities/MediaStream.cs

@@ -2,7 +2,6 @@
 #pragma warning disable CS1591
 
 using System;
-using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Globalization;