[generator] Avoid C#11 delegate cache overhead.

[jpobst]

Fixes: #1034
Context: dotnet/roslyn#62832 (comment)

On a recent .NET 7 bump, we found a ~4.5% app size regression related to a new C# 11 delegate feature.

Instead of emitting e.g.

static Delegate GetGetActionBarHandler ()
{
	if (cb_getActionBar == null)
		cb_getActionBar = JNINativeWrapper.CreateDelegate ((_JniMarshal_PP_L) n_GetActionBar);
	return cb_getActionBar;
}

generator should instead emit:

static Delegate GetGetActionBarHandler ()
{
	if (cb_getActionBar == null)
		cb_getActionBar = JNINativeWrapper.CreateDelegate (new _JniMarshal_PP_L(n_GetActionBar));
	return cb_getActionBar;
}

This should avoid a C#11 "caching method group conversion", allowing Mono.Android.dll assembly size to be ~unchanged between C#10 and C#11.

Test JI bump with this change is green: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6867774&view=results.

[jonathanpeppers]

The changes here look right.

The only thing we should do in the next java.interop bump, is delete this line:

https://github.com/xamarin/xamarin-android/blob/ab6712a48e4b93e9c3d605c7b393814d477e2bc8/src/Mono.Android/Mono.Android.csproj#L25-L26

And just let Mono.Android.dll compile with C# 11, checking what the size diff is.

[jpobst]

Yeah, my bad, I should have mentioned my test XA PR does that and no tests fail. 🎉

https://github.com/xamarin/xamarin-android/pull/7478/files