[One .NET] fix remaining Mono.Android.Export linker warning

Context: dotnet#5652

If you build an app with:

    > dotnet build -c Release -p:SuppressTrimAnalysisWarnings=false

You get the linker warning:

    src\Mono.Android\Android.Runtime\AndroidRuntime.cs(296,5): warning IL2026: Android.Runtime.AndroidTypeManager.CreateDynamicCallback(MethodInfo): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.

We can simply suppress the warning:

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

If we applied this, every app would preserve
`Mono.Android.Export.dll`, so we should not do this:

    [DynamicDependency ("Create", "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]

This is already preserved because we have a `DynamicDependency` in
`ExportAttribute.cs`:

    [DynamicDependency (DynamicallyAccessedMemberTypes.All, "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]

I also had to bring in the source for `UnconditionalSuppressMessage`
for `monoandroid10` or `netstandard2.0` assemblies. I also changed an
MSBuild `Condition`, so it will continue to work in .NET 7.

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
@@ -285,6 +286,8 @@ protected override IEnumerable<string> GetSimpleReferences (Type type)
 
 		static MethodInfo? dynamic_callback_gen;
 
+		// See ExportAttribute.cs
+		[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "Mono.Android.Export.dll is preserved when [Export] is used via [DynamicDependency].")]
 		static Delegate CreateDynamicCallback (MethodInfo method)
 		{
 			if (dynamic_callback_gen == null) {

src/Xamarin.Android.NamingCustomAttributes/System.Runtime.CompilerServices/UnconditionalSuppressMessageAttribute.cs

@@ -0,0 +1,92 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Suppresses reporting of a specific rule violation, allowing multiple suppressions on a
+    /// single code artifact.
+    /// </summary>
+    /// <remarks>
+    /// <see cref="UnconditionalSuppressMessageAttribute"/> is different than
+    /// <see cref="SuppressMessageAttribute"/> in that it doesn't have a
+    /// <see cref="ConditionalAttribute"/>. So it is always preserved in the compiled assembly.
+    /// </remarks>
+    [AttributeUsage(AttributeTargets.All, Inherited = false, AllowMultiple = true)]
+#if SYSTEM_PRIVATE_CORELIB
+    public
+#else
+    internal
+#endif
+    sealed class UnconditionalSuppressMessageAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnconditionalSuppressMessageAttribute"/>
+        /// class, specifying the category of the tool and the identifier for an analysis rule.
+        /// </summary>
+        /// <param name="category">The category for the attribute.</param>
+        /// <param name="checkId">The identifier of the analysis rule the attribute applies to.</param>
+        public UnconditionalSuppressMessageAttribute(string category, string checkId)
+        {
+            Category = category;
+            CheckId = checkId;
+        }
+
+        /// <summary>
+        /// Gets the category identifying the classification of the attribute.
+        /// </summary>
+        /// <remarks>
+        /// The <see cref="Category"/> property describes the tool or tool analysis category
+        /// for which a message suppression attribute applies.
+        /// </remarks>
+        public string Category { get; }
+
+        /// <summary>
+        /// Gets the identifier of the analysis tool rule to be suppressed.
+        /// </summary>
+        /// <remarks>
+        /// Concatenated together, the <see cref="Category"/> and <see cref="CheckId"/>
+        /// properties form a unique check identifier.
+        /// </remarks>
+        public string CheckId { get; }
+
+        /// <summary>
+        /// Gets or sets the scope of the code that is relevant for the attribute.
+        /// </summary>
+        /// <remarks>
+        /// The Scope property is an optional argument that specifies the metadata scope for which
+        /// the attribute is relevant.
+        /// </remarks>
+        public string? Scope { get; set; }
+
+        /// <summary>
+        /// Gets or sets a fully qualified path that represents the target of the attribute.
+        /// </summary>
+        /// <remarks>
+        /// The <see cref="Target"/> property is an optional argument identifying the analysis target
+        /// of the attribute. An example value is "System.IO.Stream.ctor():System.Void".
+        /// Because it is fully qualified, it can be long, particularly for targets such as parameters.
+        /// The analysis tool user interface should be capable of automatically formatting the parameter.
+        /// </remarks>
+        public string? Target { get; set; }
+
+        /// <summary>
+        /// Gets or sets an optional argument expanding on exclusion criteria.
+        /// </summary>
+        /// <remarks>
+        /// The <see cref="MessageId "/> property is an optional argument that specifies additional
+        /// exclusion where the literal metadata target is not sufficiently precise. For example,
+        /// the <see cref="UnconditionalSuppressMessageAttribute"/> cannot be applied within a method,
+        /// and it may be desirable to suppress a violation against a statement in the method that will
+        /// give a rule violation, but not against all statements in the method.
+        /// </remarks>
+        public string? MessageId { get; set; }
+
+        /// <summary>
+        /// Gets or sets the justification for suppressing the code analysis message.
+        /// </summary>
+        public string? Justification { get; set; }
+    }
+}

src/Xamarin.Android.NamingCustomAttributes/Xamarin.Android.NamingCustomAttributes.projitems

@@ -23,8 +23,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\LayoutAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Java.Interop\IJniNameProviderAttribute.cs" />
   </ItemGroup>
-  <ItemGroup Condition=" '$(TargetFramework)' != 'net6.0' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'monoandroid10' or '$(TargetFramework)' == 'netstandard2.0' ">
     <Compile Include="$(MSBuildThisFileDirectory)System.Diagnostics.CodeAnalysis/DynamicallyAccessedMemberTypes.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System.Diagnostics.CodeAnalysis/DynamicDependencyAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System.Runtime.CompilerServices/UnconditionalSuppressMessageAttribute.cs" />
   </ItemGroup>
 </Project>