(Re)move the type.ContainsGenericParameters check in JniRuntime.JniTypeManager.GetTypeSignature()?

[jonpryor]

Context: dotnet/android#9913
Context: dotnet/android@de49d96

dotnet/android#9913 updated JNIEnv.GetJniName(Type) to use JniRuntime.JniTypeManager.GetTypeSignature(Type). It promptly failed on CI:

E NUnit   :  : System.NotSupportedException : 'Java.InteropTests.GenericHolder`1[T]' contains a generic type definition. This is not supported.
E NUnit   :    at Java.Interop.JniRuntime.JniTypeManager.GetTypeSignature(Type )
E NUnit   :    at Android.Runtime.JNIEnv.GetJniName(Type )
E NUnit   :    at Java.Interop.TypeManager.RegisterType(String , Type )
E NUnit   :    at Android.Runtime.JNIEnvInit.RegisterJniNatives(IntPtr , Int32 , IntPtr , IntPtr , Int32 )
E NUnit   :    at Java.Interop.JniEnvironment.Object.AllocObject(JniObjectReference )
E NUnit   :    at Java.Interop.JniType.AllocObject()
E NUnit   :    at Java.Interop.JniPeerMembers.JniInstanceMethods.StartCreateInstance(String , Type , JniArgumentValue* )
E NUnit   :    at Java.Lang.Object..ctor()
E NUnit   :    at Java.InteropTests.GenericHolder`1[[System.Int32, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]]..ctor()
E NUnit   :    at Java.InteropTests.JnienvTest.NewClosedGenericTypeWorks()
E NUnit   :    at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
E NUnit   :    at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object , BindingFlags )

because of this check:

java-interop/src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

Lines 148 to 149 in 19ec1a9

	if (type.ContainsGenericParameters)
	throw new NotSupportedException ($"'{type}' contains a generic type definition. This is not supported.");

Should we have that check, first?

We certainly allow generic types to be Java.Lang.Object subclasses, e.g. GenericHolder<T>! Constructing instances of such types need to go through the type manager; should .GetTypeSignature() really be asserting that generic type parameters not exist?

Or is it because we're using the type definition? typeof(GenericHolder<int>).ContainsGenericParameters is false, i.e. that would pass the check; it's typeof(GenericHolder<>).ContainsGenericParameters which fails.

Indeed, it's because between JniEnvironment.Object.AllocObject() and JNIEnvInit.RegisterJniNatives() is the static constructor for the Java Callable Wrapper for GenericHolder<T>, which must implicitly use the type definition, as it cannot know what type parameters will be used:

public /* partial */  class GenericHolder_1
{
	public static final String __md_methods;
	static {
		__md_methods = "";
		mono.android.Runtime.register ("Java.InteropTests.GenericHolder`1, Mono.Android-Tests", GenericHolder_1.class, __md_methods);
	}
}

Given that we later use a generic type definition:

java-interop/src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

Lines 160 to 170 in 19ec1a9

	var genericDef = isGeneric ? type.GetGenericTypeDefinition () : type;
	if (isGeneric) {
	if (genericDef == typeof (JavaArray<>) || genericDef == typeof (JavaObjectArray<>)) {
	var r = GetTypeSignature (type.GenericTypeArguments [0]);
	return r.AddArrayRank (rank + 1);
	}
	var genericSimpleRef = GetSimpleReference (genericDef);
	if (genericSimpleRef != null)
	return new JniTypeSignature (genericSimpleRef, rank, false);
	}

we can probably just remove the check entirely.