[generator] restore [RegisterAttribute] for *Implementor types

[jonathanpeppers]

Fixes: dotnet/android#8337

A customer's app with the code:

SetContentView(Resource.Layout.activity_main);
FindViewById<Button>(Resource.Id.asd).Click += MainActivity_Click;

Crashes with -c Release -p:AndroidLinkMode=r8 with:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.companyname.New_folder/crc64abe8cc9139195b67.MainActivity}: java.lang.ClassNotFoundException: android.view.View_IOnClickListenerImplementor
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3644)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3781)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:101)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:138)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2306)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loopOnce(Looper.java:201)
    at android.os.Looper.loop(Looper.java:288)
    at android.app.ActivityThread.main(ActivityThread.java:7918)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)
Caused by: java.lang.ClassNotFoundException: android.view.View_IOnClickListenerImplementor
    at crc64abe8cc9139195b67.MainActivity.n_onCreate(Native Method)
    at crc64abe8cc9139195b67.MainActivity.onCreate(MainActivity.java:30)
    at android.app.Activity.performCreate(Activity.java:8342)
    at android.app.Activity.performCreate(Activity.java:8321)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1417)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3625)
    ... 12 more

Which, can be solved by adding your own proguard rules like:

-keep class android.view.View_IOnClickListenerImplementor { *; }

In a91ae7f, we thought (incorrectly):

Additionally, stop emitting the [Register] attribute for
*Implementor classes:

[global::Android.Runtime.Register ("mono/android/view/View_OnFocusChangeListenerImplementor")]
partial class IOnFocusChangeListenerImplementor {/* … */}

The [Register] attribute is not needed, because *Implementor
classes are generated internal implementation details.

proguard_xamarin.cfg has the entry:

-keep class mono.android.** { *; <init>(...); }

So we could do android.**, but that would certainly preserve way too much!

For now, let's just restore [Register] to revisit this issue at a later date -- maybe .NET 9?

[jonathanpeppers]

Added a test at: dotnet/android#8407