[tvos][mediaplayer] Enable MPContentItem on tvOS

No news from Apple on the original rdar but the issue was fixed in Xcode
8 beta 6 (and likely earlier betas)

> MP_EXTERN_CLASS_AVAILABLE(3_0)
> @interface MPMediaItemArtwork : NSObject

which makes it possible to enable the type (as it does nor depend on
other, unavailable, types).

references:
* rdar://24982126
* https://trello.com/c/2gxuFbeS

src/mediaplayer.cs

@@ -1314,7 +1314,6 @@ namespace XamCore.MediaPlayer {
 		NSString PropertyIsLiveStream { get; }
 	}
 
-	[NoTV] // This type was made available in tvOS but MPMediaItemArtwork is still restricted radar://24982126 https://trello.com/c/2gxuFbeS
 	[Since (7,1)]
 	[BaseType (typeof (NSObject))]
 	[DisableDefaultCtor] // crash if used
@@ -1340,10 +1339,12 @@ namespace XamCore.MediaPlayer {
 		string Title { get; set; }
 
 		[iOS (10,0)]
+		[TV (10,0)]
 		[Export ("streamingContent")]
 		bool StreamingContent { [Bind ("isStreamingContent")] get; set; }
 
 		[iOS (10,0)]
+		[TV (10,0)]
 		[Export ("explicitContent")]
 		bool ExplicitContent { [Bind ("isExplicitContent")] get; set; }