[Java.Interop.Tools.TypeNameMappings] fix trimmer warnings

[jonathanpeppers]

Context: https://github.com/xamarin/xamarin-android/blob/e987ac458536e59a8329a06d5c5d5f4d4ea2c6b6/src/Mono.Android/Mono.Android.csproj#L69-L71

In xamarin/xamarin-android, we import the source for Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs, and unfortunately there are some trimmer warnings:

external\Java.Interop\src\Java.Interop.Tools.TypeNameMappings\Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs(182,9):
error IL2070: 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.Interfaces' in call to 'System.Type.GetInterfaces()'. The parameter 'type' of method 'Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToJniName(Type, ExportParameterKind)' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to.

From the code:

if (!type.GetInterfaces ().Any (t => t.FullName == "Android.Runtime.IJavaObject"))

It appears we can instead look for IJavaPeerable and use trim-safe behavior instead:

if (Type.GetType ("Java.Interop.IJavaPeerable, Java.Interop", throwOnError: true)
    .IsAssignableFrom (type))

I also cached the Type.GetType() call with Lazy<T>.

However, in some cases we run this code under an MSBuild context. In
this case, we don't have Java.Interop.dll to load, so we should
instead use:

type.GetInterfaces ().Any (t => t.FullName == "Java.Interop.IJavaPeerable");

To catch warnings in this project going forward:

• Multi-target to netstandard2.0 and net8.0 using $(DotNetTargetFramework). netstandard2.0 is used for MSBuild-task assemblies inside VS.

• Enable trimmer warnings for net8.0

As I targeted net8.0, various null-reference-type warnings appeared, which I also fixed.

[jonathanpeppers]

The net8.0 build failed without this:

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=9093665&view=logs&j=4ab9874f-3b82-50ef-2a6b-4340d0043621&t=1d9063ca-c961-5752-dc23-cb1130ee3d46&l=1141

Which is kind of odd, I thought that was the default in net5.0+? We should use it anyway to make incremental builds quicker.

[jonathanpeppers]

/cc @simonrozsival @vitek-karas