Make GetCustomAttribute calls optional in JniTypeManager

Optionally do not look for the `JniAddNativeMethodRegistrationAttribute`.
It turns out that this attribute is not commonly present in
Xamarin.Android applications and so all of the calls to
`GetCustomAttribute` failed to find it, incurring unnecessary
performance penalty on the application.

Make the code consult a global flag (in case of Xamarin.Android it is
set on the build time) which tells it whether or not to skip querying
for the above custom attribute. The flag is `true` by default to
maintain backward compatibility.

Author: grendello

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -286,7 +286,7 @@ protected bool TryRegisterNativeMembers (JniType nativeClass, Type type, string?
 
 			static Type [] registerMethodParameters = new Type [] { typeof (JniNativeMethodRegistrationArguments) };
 
-			static bool TryLoadJniMarshalMethods (JniType nativeClass, Type type, string? methods)
+			bool TryLoadJniMarshalMethods (JniType nativeClass, Type type, string? methods)
 			{
 				var marshalType = type?.GetNestedType ("__<$>_jni_marshal_methods", BindingFlags.NonPublic);
 				if (marshalType == null)
@@ -299,7 +299,7 @@ static bool TryLoadJniMarshalMethods (JniType nativeClass, Type type, string? me
 
 			static List<JniNativeMethodRegistration> sharedRegistrations = new List<JniNativeMethodRegistration> ();
 
-			static bool TryRegisterNativeMembers (JniType nativeClass, Type marshalType, string? methods, MethodInfo? registerMethod)
+			bool TryRegisterNativeMembers (JniType nativeClass, Type marshalType, string? methods, MethodInfo? registerMethod)
 			{
 				bool lockTaken = false;
 				bool rv = false;
@@ -331,8 +331,11 @@ static bool TryRegisterNativeMembers (JniType nativeClass, Type marshalType, str
 				return rv;
 			}
 
-			static bool FindAndCallRegisterMethod (Type marshalType, JniNativeMethodRegistrationArguments arguments)
+			bool FindAndCallRegisterMethod (Type marshalType, JniNativeMethodRegistrationArguments arguments)
 			{
+				if (!Runtime.Options.JniAddNativeMethodRegistrationAttributePresent)
+					return false;
+
 				bool found = false;
 
 				foreach (var methodInfo in marshalType.GetRuntimeMethods ()) {

src/Java.Interop/Java.Interop/JniRuntime.cs

@@ -63,6 +63,7 @@ public partial class CreationOptions {
 			public  JniObjectReferenceManager?  ObjectReferenceManager      {get; set;}
 			public  JniTypeManager?             TypeManager                 {get; set;}
 			public  string?                     JvmLibraryPath              {get; set;}
+			public  bool                        JniAddNativeMethodRegistrationAttributePresent { get; set; } = true;
 
 			public CreationOptions ()
 			{
@@ -182,11 +183,11 @@ public static void SetCurrent (JniRuntime newCurrent)
 
 		internal    bool                                TrackIDs            {get; private set;}
 		internal    bool                                NewObjectRequired   {get; private set;}
+		internal    CreationOptions                     Options             { get; }
 
 		protected JniRuntime (CreationOptions options)
 		{
-			if (options == null)
-				throw new ArgumentNullException (nameof (options));
+			Options = options ?? throw new ArgumentNullException (nameof (options));
 			if (options.InvocationPointer == IntPtr.Zero)
 				throw new ArgumentException ("options.InvocationPointer is null", nameof (options));