[nativeaot] introduce Microsoft.Android.Runtime.NativeAOT.dll

[jonathanpeppers]

This moves:

• C# code from samples/NativeAOT to a new assembly, Microsoft.Android.Runtime.NativeAOT.dll.

• Java code from samples/NativeAOT to Xamarin.Android.Build.Tasks, to be generated at build time.

• C# code from Java.Interop.Environment.csproj has been trimmed down and moved to Microsoft.Android.Runtime.NativeAOT.dll as well.

• A new TypeMappingStep implements the simplest MVP for a "typemap", replacing the NativeAotTypeManager.InitializeTypeMappings() method.

[jonathanpeppers]

Affected tests look green:

[jonathanpeppers]

There are some APK test failures, but look network-related:

System.AggregateException : AggregateException_ctor_DefaultMessage (net_http_message_not_success_statuscode_reason, 403, Forbidden) (net_http_message_not_success_statuscode_reason, 403, Forbidden)

[jonpryor]

This likewise should be in the static constructor.

Alternatively, TypeMappings should be an instance field.

[jonathanpeppers]

I made it an instance field, as I think it makes more sense for the instance of the *TypeManager to hold its typemap.

[jonpryor]

Draft commit message:

This moves:
* C# code from `samples/NativeAOT` to a new assembly,
`Microsoft.Android.Runtime.NativeAOT.dll`.
* Java code from `samples/NativeAOT` to
`Xamarin.Android.Build.Tasks`, to be generated at build time.
* C# code from `Java.Interop.Environment.csproj` has been trimmed
down, copied into `Microsoft.Android.Runtime.NativeAOT.dll`.
* Minimum Viable Product™ "typemap" implementation.
* Change namespace to `Microsoft.Android.Runtime`## Minimum Viable Product™ typemaps
A quick-and-dirty -- and totally unoptimized -- typemap implementation
has been added as a new `TypeMappingStep` into
`Microsoft.Android.Sdk.ILLink`.
Given this initial starting point within `NativeAotTypeManager`:
partial class NativeAotTypeManager { IDictionary<string, Type> TypeMappings = new Dictionary<string, Type>(StringComparer.Ordinal); NativeAotTypeManager () {	InitializeTypeMappings (); } void InitializeTypeMappings () { throw new InvalidOperationException ("Should be replaced at build time!"); }}
then at build time `TypeMappingStep` will *replace* the body of
`NativeAotTypeManager.InitializeTypeMappings()` with repeated
`IDictionary<string, Type>.Add()` calls for each type which survived
linking:
// Cecil-generated code, C# equivalentvoid InitializeTypeMappings (){ TypeMappings.Add ("java/lang/Object", typeof (Java.Lang.Object)); TypeMappings.Add ("android/app/Application", typeof (Android.App.Application)); // …}