[jcw-gen] Use + for nested types, not /

[jonpryor]

TODO: full explanation.

#1302 and dotnet/android#9750 has turned into a bit of a boondoggle. Maybe the better approach is to just update jcw-gen to use + instead of / for nested types, instead of both jcw-gen and generator, because the generator changes just require changes everywhere.

Will this smaller fix work?

[jonpryor]

/azp run

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[jonpryor]

Draft commit message:

[jcw-gen] Use `+` for nested types, not `/`

Context: https://github.com/dotnet/android/pull/9747
Context: https://discord.com/channels/732297728826277939/732297837953679412/1336353039031734352
Context: https://discord.com/channels/732297728826277939/732297837953679412/1336358257769316372
Context: https://github.com/dotnet/java-interop/pull/1302
Context: https://github.com/dotnet/android/pull/9750

The `[Register]` attribute provides "connector method" names, and for
interface methods this will also include the name of the type which
declares the method, which itself may be in a nested type:

	namespace Android.App {
	  public partial class Application {
	    public partial interface IActivityLifecycleCallbacks : IJavaObject, IJavaPeerable {
	      [Register (
	          name: "onActivityCreated",
	          signature: "(Landroid/app/Activity;Landroid/os/Bundle;)V",
	          connector: "GetOnActivityCreated_Landroid_app_Activity_Landroid_os_Bundle_Handler:Android.App.Application/IActivityLifecycleCallbacksInvoker, Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null")]
	      void OnActivityCreated (Android.App.Activity activity, Android.OS.Bundle? savedInstanceState);

	      // …
	    }
	  }
	}

The `connector` value is copied as-is into Java Callable Wrappers,
as part of the `__md_methods` value and `Runtime.register()` call.
Given the C# type:

	partial class MauiApplication : Application {
	  partial class ActivityLifecycleCallbacks : Java.Lang.Object, Application.IActivityLifecycleCallbacks {
	    public void OnActivityCreated (Activity activity, Bundle? savedInstanceState) => …
	  }
	}

then `Java.Interop.Tools.JavaCallableWrappers` will produce:

	// Java Callable Wrapper
	/* partial */ class MauiApplication_ActivityLifecycleCallbacks
	{
	  public static final String __md_methods;
	  static {
	    __md_methods =
	      // …
	      "n_onActivityCreated:(Landroid/app/Activity;Landroid/os/Bundle;)V:GetOnActivityCreated_Landroid_app_Activity_Landroid_os_Bundle_Handler:Android.App.Application/IActivityLifecycleCallbacksInvoker, Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null\n" +
	      // …
	      "";
	    mono.android.Runtime.register ("Microsoft.Maui.MauiApplication+ActivityLifecycleCallbacks, Microsoft.Maui", MauiApplication_ActivityLifecycleCallbacks.class, __md_methods);
	  }
	}

The `signature` and `connector` values from the `[Register(…)]` on
the method declaration are copied as-is into `__md_methods`.
As the `connector` value contains a `/`, `__md_methods` does as well.

This has worked fine for nearly 15+ years…on Mono/MonoVM.

This *fails* on NativeAOT and CoreCLR, as:

	Type.GetType ("Android.App.Application/IActivityLifecycleCallbacksInvoker, Mono.Android, …", throwOnError:true)

fails with:

	TypeLoadException: Could not resolve type 'Android.App.Application/IActivityLifecycleCallbacksInvoker' in assembly 'Mono.Android, …'.

The reason for the failure is that when using Reflection APIs such as
`Type.GetType()`, the [`Type.AssemblyQualifiedName` grammar][0] must
be followed, and that grammar uses `+` to "Precede a nested class",
*not* `/`.  (`/` isn't even in the Reflection grammar!)

(Aside: where does `/` come from then?  It's the *IL* separator for
nested types!)

For eventual CoreCLR and NativeAOT support, then, we need to replace
`/` with `+` *somewhere* before it hits `Type.GetType()`.

There are (at least?) three places to do so:

 1. Within `JniRuntime.JniTypeManager.RegisterNativeMembers()` or
    equivalent override.

 2. Within `generator`, updating the contents of `[Register]`.

 3. Within `Java.Interop.Tools.JavaCallableWrappers`.

(1) is rejected out of hand as it would be additional work done on-
device at runtime.  Why do that if we don't need to?

(2) was attempted in dotnet/java-interop#1302 & dotnet/android#9750.
It turned into a bit of a boondoggle, because there are lots of
linker steps which interpret the `connector` value on `[Register]`,
and it "just worked" that the `connector` value contained IL names,
as the linker steps deal in IL!  Trying to update `connector` to
instead contain Reflection syntax required finding all the places in
the linker that used `connector` values, which was tedious & brittle.

"Just" (2) is also inadequate, as it would require new binding
assemblies to take advantage of, so (2) *also* needed (3).

Which brings us to (3), the current approach: *don't* alter the
semantics of the `connect` value within `[Register]`, and instead
require that `Java.Interop.Tools.JavaCallableWrappers` replace all
instances of `/` with `+` within `__md_methods`.  This is needed
*anyway*, for compatibility with existing bindings, and also avoids
lots of pain that (2) encountered.

With this approach, `generator` output is unchanged, and
`jcw-gen` output for `MauiApplication.ActivityLifecycleCallbacks`
becomes:

	// Java Callable Wrapper
	/* partial */ class MauiApplication_ActivityLifecycleCallbacks
	{
	  public static final String __md_methods;
	  static {
	    __md_methods =
	      // …
	      "n_onActivityCreated:(Landroid/app/Activity;Landroid/os/Bundle;)V:GetOnActivityCreated_Landroid_app_Activity_Landroid_os_Bundle_Handler:Android.App.Application+IActivityLifecycleCallbacksInvoker, Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null\n" +
	      // …
	      "";
	    mono.android.Runtime.register ("Microsoft.Maui.MauiApplication+ActivityLifecycleCallbacks, Microsoft.Maui", MauiApplication_ActivityLifecycleCallbacks.class, __md_methods);
	  }
	}

[0]: https://learn.microsoft.com/en-us/dotnet/api/system.type.assemblyqualifiedname?view=net-9.0#remarks

[jonpryor]

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