[marshal methods] Make AndroidEnvironmentInternal.UnhandledException public

Fixes: #10602
Context: 8bc7a3e

This is in the category of "how the hell has it worked before?", as every time the
`AndroidEnvironmentInternal.UnhandledException` method living in `Mono.Android.Runtime.dll`
would be called, the runtime would respond with a method not accessible exception, rightly
so as both the type and the method were internal and visible only to `Mono.Android.dll`.

I guess we haven't had many unhandled exceptions in marshal method wrappers over the years?

This PR fixes the problem by making both the type and the method public. This is the simplest
way to fix the issue.

Author: grendello

src/Mono.Android.Runtime/Android.Runtime/AndroidEnvironmentInternal.cs

@@ -2,11 +2,11 @@
 
 namespace Android.Runtime
 {
-	internal static class AndroidEnvironmentInternal
+	public static class AndroidEnvironmentInternal
 	{
 		internal static Action<Exception>? UnhandledExceptionHandler;
 
-		internal static void UnhandledException (Exception e)
+		public static void UnhandledException (Exception e)
 		{
 			if (UnhandledExceptionHandler == null) {
 				return;