Additional fixes and improvements to JavaClassWrapper

- Fix crashing bug when invoking class constructor with parameters
- Add support for accessing class constants
- Add support for Godot Callable arguments. A Godot Callable can be wrapped by a Java Runnable to allow Java logic to run arbitrary Godot lambdas
- Automatically convert java.lang.CharSequence to Godot String as needed
- Code cleanup

platform/android/plugin/godot_plugin_jni.cpp

@@ -33,7 +33,6 @@
 #include "api/java_class_wrapper.h"
 #include "api/jni_singleton.h"
 #include "jni_utils.h"
-#include "string_android.h"
 
 #include "core/config/engine.h"
 #include "core/error/error_macros.h"
@@ -136,5 +135,10 @@ JNIEXPORT void JNICALL Java_org_godotengine_godot_plugin_GodotPlugin_nativeEmitS
 	}
 
 	singleton->emit_signalp(StringName(signal_name), args, count);
+
+	// Manually invoke the destructor to decrease the reference counts for the variant arguments.
+	for (int i = 0; i < count; i++) {
+		variant_params[i].~Variant();
+	}
 }
 }