[generator] improve *Implementor constructors

[jonathanpeppers]

Context: dotnet/maui#12130
Context: https://github.com/angelru/CvSlowJittering

Profiling a .NET MAUI customer sample while scrolling on a Pixel 5, I see ~2.2% of the time spent in:

(2.2%) mono.android!Android.Views.View.IOnFocusChangeListenerImplementor..ctor()

https://github.com/dotnet/maui/blob/2041476e78452891029f4d2bd806c45be42f4878/src/Core/src/Handlers/View/ViewHandler.Android.cs#L14

MAUI subscribes to Android.Views.View.FocusChange for every view placed on the screen -- which happens while scrolling in this sample.

Reviewing, the generated code for this constructor still uses the outdated JNIEnv APIs:

public IOnFocusChangeListenerImplementor () : base (global::Android.Runtime.JNIEnv.StartCreateInstance ("mono/android/view/View_OnFocusChangeListenerImplementor", "()V"), JniHandleOwnership.TransferLocalRef)
{
    global::Android.Runtime.JNIEnv.FinishCreateInstance (((global::Java.Lang.Object) this).Handle, "()V");
}

Which we can change to use the newer/faster Java.Interop APIs:

public unsafe IOnFocusChangeListenerImplementor () : base (IntPtr.Zero, JniHandleOwnership.DoNotTransfer)
{
    const string __id = "()V";
    if (((global::Java.Lang.Object) this).Handle != IntPtr.Zero)
        return;
    var h = JniPeerMembers.InstanceMethods.StartCreateInstance (__id, ((object) this).GetType (), null);
    SetHandle (h.Handle, JniHandleOwnership.TransferLocalRef);
    JniPeerMembers.InstanceMethods.FinishCreateInstance (__id, this, null);
}

These are better because the equivalent call to JNIEnv.FindClass() is cached among other things.

After these changes, I instead see:

(0.81%) mono.android!Android.Views.View.IOnFocusChangeListenerImplementor..ctor()

This should improve the performance of all C# events that wrap Java listeners -- and all .NET MAUI views on Android.

These changes also stop emitting the [Register] attribute for *Implementor classes:

[global::Android.Runtime.Register ("mono/android/view/View_OnFocusChangeListenerImplementor")]

Which is not needed, because *Implementor classes are generated, internal implementation details.

[jpobst]

I approve that the generator changes look good. I will leave the JNI changes to jonp.