[Mono.Android] fix a set of the "easiest" trimmer warnings

[jonathanpeppers]

Context: #5652

Introduce a new trim-analyzers.targets file to be used in this repo:

1. Enables appropriate analyzers.

2. $(ILLinkTreatWarningsAsErrors) only does anything in builds that would run ILLink, such as an application build. Useful to be here.

3. List all the explicit ILLink and AOT warnings, so that they trigger build errors.

Eventually we should be able to import this file for Mono.Android.csproj.

Fixing the following so far:

TypeConverter

Context: https://source.dot.net/#System.ComponentModel.TypeConverter/System/ComponentModel/TypeConverter.cs,226

src\Mono.Android\System.Drawing\SizeFConverter.cs(121,49):
error IL2046: Base member 'System.ComponentModel.TypeConverter.GetProperties(ITypeDescriptorContext, Object, Attribute[])' with 'RequiresUnreferencedCodeAttribute' has a derived member 'System.Drawing.SizeFConverter.GetProperties(ITypeDescriptorContext, Object, Attribute[])' without 'RequiresUnreferencedCodeAttribute'. 'RequiresUnreferencedCodeAttribute' annotations must match across all interface implementations or overrides.

Various TypeConverter implementations need to specify [RequiresUnreferencedCode] to match the base type.

ColorValueMarshaler & IJavaObjectValueMarshaler

Context: dotnet/java-interop@7d1e705

From the trimmer warnings solved in Java.Interop.dll, we need to bring these changes forward to any *Marshaler types.

AndroidClientHandler

'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.NonPublicFields' in call to 'System.Type.GetField(String, BindingFlags)'. The return value of method 'System.Collections.IEnumerator.Current.get' 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.

This class had a loop that was not trimming safe:

for (var type = GetType (); type != null; type = type.BaseType) {
    field = type.GetField (fieldName, BindingFlags.Instance | BindingFlags.NonPublic);
    if (field != null)
        break;
}

If we look at the actual base types of AndroidClientHandler, we can simplify this loop to something that is trimming safe:

const BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic;
FieldInfo? field = typeof (HttpClientHandler).GetField (fieldName, flags) ??
    typeof (HttpMessageHandler).GetField (fieldName, flags);

There should be no need to iterate beyond HttpMessageHandler, the code is looking for this field:

https://github.com/dotnet/runtime/blob/135fec006e727a31763271984cd712f1659ccbd3/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.cs#L25

[jonathanpeppers]

/cc @simonrozsival @vitek-karas