[generator] Wrap SupportedOSPlatformAttribute in NET (#779)

Fixes: #777

Commit da12df4 updated `generator` to emit and use a
`SupportedOSPlatformAttribute`, wrapping it in `NET5_0_OR_GREATER`
to prevent it from being defined in .NET 6:

	#if !NET5_0_OR_GREATER
	namespace System.Runtime.Versioning {
	    partial class SupportedOSPlatformAttribute {
	    }
	}
	#endif

`NET5_0_OR_GREATER` was chosen because, at the time, we believed that
it would be defined when building code targeting .NET 5+.

Unfortunately, this is not yet the case; it's only in the
[design stage][0].

Consequently, when building on xamarin-android CI,
[unit tests fail][1] because the `SupportedOSPlatformAttribute` type
is defined when it shouldn't be defined:

	…/DotNetBuildandroid.21-armandroid.21-arm64android.21-x86android.21-x64False/obj/Debug/net6.0-android/UnnamedProject.AssemblyInfo.cs(21,38):
	warning CS0436: The type 'SupportedOSPlatformAttribute' in '…/DotNetBuildandroid.21-armandroid.21-arm64android.21-x86android.21-x64False/obj/Debug/net6.0-android/generated/src/__NamespaceMapping__.cs'
	conflicts with the imported type 'SupportedOSPlatformAttribute' in 'System.Runtime, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
	Using the type defined in '…/DotNetBuildandroid.21-armandroid.21-arm64android.21-x86android.21-x64False/obj/Debug/net6.0-android/generated/src/__NamespaceMapping__.cs'.

(It's a warning, but the containing test asserts that there are no
build warnings, hence the failing unit test.)

Instead, wrap `SupportedOSPlatformAttribute` in `NET`, which is
currently defined when building code for .NET 5+, but *not* defined
when building code targeting the Xamarin.Android target frameworks or
the `net472` target framework moniker.

	#if !NET
	namespace System.Runtime.Versioning {
	    partial class SupportedOSPlatformAttribute {
	    }
	}
	#endif

[0]: https://github.com/dotnet/designs/blob/6149aa6df1567d4d69e1219247a1e423b775457f/accepted/2020/or-greater-defines/or-greater-defines.md
[1]: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4370096&view=ms.vss-test-web.build-test-results-tab&runId=18107834&resultId=100055&paneView=attachments

Author: radekdoulik

tools/generator/Java.Interop.Tools.Generator.ObjectModel/NamespaceMapping.cs

@@ -35,7 +35,7 @@ public void Generate (CodeGenerationOptions opt, GenerationInfo gen_info)
 				// [SupportedOSPlatform] only exists in .NET 5.0+, so we need to generate a
 				// dummy one so earlier frameworks can compile.
 				if (opt.CodeGenerationTarget == Xamarin.Android.Binder.CodeGenerationTarget.XAJavaInterop1) {
-					sw.WriteLine ("#if !NET5_0_OR_GREATER");
+					sw.WriteLine ("#if !NET");
 					sw.WriteLine ("namespace System.Runtime.Versioning {");
 					sw.WriteLine ("    [System.Diagnostics.Conditional(\"NEVER\")]");
 					sw.WriteLine ("    [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Event | AttributeTargets.Method | AttributeTargets.Module | AttributeTargets.Property | AttributeTargets.Struct, AllowMultiple = true, Inherited = false)]");