[Java.Interop] JavaTypeParametersAttribute only in net6

Context: dotnet/android#6549

When attempting to bump xamarin-android to use bc5bcf4, the build
failed because bc5bcf4 added a "duplicate" type
`Java.Interop.JavaTypeParametersAttribute`:

	error CS0433: The type 'JavaTypeParametersAttribute' exists in both
	'Java.Interop, Version=0.1.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065' and
	'Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065'.

The plan was to use type forwarders for this type from
`Mono.Android.dll` to `Java.Interop.dll`, but in retrospect this
sounds like a potential "forward-incompatible ABI break" (e56a8c8)
as assemblies built against the newer `Mono.Android.dll` (with type
forwarders) won't be usable on previous Xamarin.Android SDKs.

Make `JavaTypeParametersAttribute` conditional on `NET`, so that it's
only included in .NET 6 builds.  .NET SDK for Android will then use
type forwarders, which won't be an issue as there's no previous .NET
release to be forward compatible with.

Update `tests/generator-Tests` so that `JavaTypeParametersAttribute`
is once again provided for non-`NET` builds.

Author: jonpryor

src/Java.Interop/Java.Interop/JavaTypeParametersAttribute.cs

@@ -1,5 +1,7 @@
 using System;
 
+#if NET
+
 namespace Java.Interop
 {
 	[AttributeUsage (AttributeTargets.Class | AttributeTargets.Interface | AttributeTargets.Method,
@@ -14,3 +16,5 @@ public JavaTypeParametersAttribute (string [] typeParameters)
 		public string [] TypeParameters { get; }
 	}
 }
+
+#endif  // NET

tests/generator-Tests/Integration-Tests/Compiler.cs

@@ -40,6 +40,9 @@ public static Assembly Compile (Xamarin.Android.Binder.CodeGeneratorOptions opti
 			if (options.CodeGenerationTarget == CodeGenerationTarget.JavaInterop1) {
 				preprocessorSymbols.Add ("JAVA_INTEROP1");
 			}
+#if NET
+			preprocessorSymbols.Add ("NET");
+#endif  // NET
 
 			var parseOptions = new CSharpParseOptions (preprocessorSymbols:preprocessorSymbols);
 

tests/generator-Tests/SupportFiles/JavaTypeParametersAttribute.cs

@@ -0,0 +1,18 @@
+#if !NET
+
+using System;
+
+namespace Java.Interop
+{
+	public class JavaTypeParametersAttribute : Attribute
+	{
+		public JavaTypeParametersAttribute (string [] typeParameters)
+		{
+			TypeParameters = typeParameters;
+		}
+
+		public string [] TypeParameters { get; set; }
+	}
+}
+
+#endif  // !NET