[Java.Interop] fix .NET 6 linker warnings (#870)

Context: dotnet/android#5652

If you build a .NET 6 Android app:

	dotnet new android
	dotnet build -c Release -bl -p:SuppressTrimAnalysisWarnings=false

You get warnings like:

	external\Java.Interop\src\Java.Interop\Java.Interop\JniRuntime.JniMarshalMemberBuilder.cs(53,4): warning IL2026: Java.Interop.JniRuntime.SetMarshalMemberBuilder(JniRuntime.CreationOptions): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.

Adding this is not sufficient to fix the warning:

	[DynamicDependency (DynamicallyAccessedMemberTypes.PublicParameterlessConstructor, "Java.Interop.MarshalMemberBuilder", "Java.Interop.Export")]

You also need to suppress [warning `IL2026`][0]:

	[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "DynamicDependency should preserve the constructor.")]

Then, because `Java.Interop.Export.dll` is not always included in
apps [^0], we also need to suppress [warning IL2035][1]:

	[UnconditionalSuppressMessage ("Trimming", "IL2035", Justification = "Java.Interop.Export.dll is not always present.")]

[0]: https://docs.microsoft.com/en-us/dotnet/core/deploying/trim-warnings/il2026
[1]: https://github.com/mono/linker/blob/66fd7119cd5744dd8bd37442ac74d2a326085406/docs/error-codes.md#il2035-unresolved-assembly-assemblyname-in-dynamicdependencyattribute

[^0]: `Java.Interop.Export.dll` is not shipped as in Xamarin.Android
      as a "MonoAndroid-profile" assembly -- and likely shouldn't be
      shipped *at all* right now -- and thus will only reliably exist
      for the few people building xamarin/Java.Interop.

src/Java.Interop/Java.Interop/JniRuntime.JniMarshalMemberBuilder.cs

@@ -6,6 +6,7 @@
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
+using System.Runtime.CompilerServices;
 using System.Text;
 
 namespace Java.Interop {
@@ -27,6 +28,11 @@ public  JniMarshalMemberBuilder        MarshalMemberBuilder        {
 		}
 
 		[System.Diagnostics.CodeAnalysis.SuppressMessage ("Design", "CA1031:Do not catch general exception types", Justification = "the *.Export assemblies are optional, so we don't care when they cannot be loaded (they are presumably missing)")]
+#if NET
+		[DynamicDependency (DynamicallyAccessedMemberTypes.PublicParameterlessConstructor, "Java.Interop.MarshalMemberBuilder", "Java.Interop.Export")]
+		[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "DynamicDependency should preserve the constructor.")]
+		[UnconditionalSuppressMessage ("Trimming", "IL2035", Justification = "Java.Interop.Export.dll is not always present.")]
+#endif
 		partial void SetMarshalMemberBuilder (CreationOptions options)
 		{
 			if (!options.UseMarshalMemberBuilder) {