Adding UnconditionalSuppressMessage doesn't solve IL2104 for entire assembly

In https://github.com/xamarin/xamarin-android/pull/6270, I'm trying to solve some of the remaining ILLink warnings we have:

```
> dotnet new android
> dotnet build -c Release -p:SuppressTrimAnalysisWarnings=false
...
src\Mono.Android\Android.Runtime\ResourceIdManager.cs(37,6): warning IL2026: Android.Runtime.ResourceIdManager.GetResourceTypeFromAssembly(Assembly): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.
src\Mono.Android\Java.Interop\JavaObjectExtensions.cs(136,4): warning IL2026: Java.Interop.JavaObjectExtensions.GetHelperType(Type,String): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.
src\Mono.Android\Java.Interop\JavaObjectExtensions.cs(131,5): warning IL2026: Java.Interop.JavaObjectExtensions.GetHelperType(Type,String): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.
Java.Interop.dll warning IL2104: Assembly 'Java.Interop' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries
Mono.Android.dll warning IL2104: Assembly 'Mono.Android' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries
```

I added `UnconditionalSuppressMessage` such as:
```csharp
[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "Types in Resource.designer.cs are preserved, because it is the root assembly passed to the linker.")]
[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "*Invoker types are preserved by the MarkJavaObjects linker step.")]
```
Both cases there isn't actually a problem here. So we just want to suppress the warnings.

`UnconditionalSuppressMessage` caused the above warnings to go away, but the warnings for the assemblies remain:
```
Java.Interop.dll warning IL2104: Assembly 'Java.Interop' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries
Mono.Android.dll warning IL2104: Assembly 'Mono.Android' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries
```

Is there an issue here, where `IL2104` is displayed and shouldn't be? Or do we need to do something else here? Thanks!