Add missing Android/tvOS optimization for OperatingSystem.IsOSPlatform() (#68313)

We were still calling into the PAL to get the platform name string, but we can just hardcode the result like we do on other platforms.

src/libraries/System.Private.CoreLib/src/System/OperatingSystem.cs

@@ -9,7 +9,7 @@ namespace System
 {
     public sealed class OperatingSystem : ISerializable, ICloneable
     {
-#if TARGET_UNIX && !TARGET_OSX && !TARGET_MACCATALYST && !TARGET_IOS
+#if TARGET_UNIX && !TARGET_OSX && !TARGET_MACCATALYST && !TARGET_IOS && !TARGET_TVOS && !TARGET_ANDROID
         private static readonly string s_osPlatformName = Interop.Sys.GetUnixName();
 #endif
 
@@ -99,6 +99,10 @@ public static bool IsOSPlatform(string platform!!)
             return platform.Equals("MACCATALYST", StringComparison.OrdinalIgnoreCase) || platform.Equals("IOS", StringComparison.OrdinalIgnoreCase);
 #elif TARGET_IOS
             return platform.Equals("IOS", StringComparison.OrdinalIgnoreCase);
+#elif TARGET_TVOS
+            return platform.Equals("TVOS", StringComparison.OrdinalIgnoreCase);
+#elif TARGET_ANDROID
+            return platform.Equals("ANDROID", StringComparison.OrdinalIgnoreCase);
 #elif TARGET_UNIX
             return platform.Equals(s_osPlatformName, StringComparison.OrdinalIgnoreCase);
 #else