react-native-reanimated Fix layout animations crash when hiding RN modal

Description

Fixes a crash occurring when using layout animations and hiding RN modal.

Fixes #3286

Changes

Firstly make a copy of _toBeRemovedRegister dictionary, mutate it while iterating through original and then assign a copy to the original.

Test code and steps to reproduce

Open and close a modal a few times. Doesn't reproduce on expo snack 🤔

import * as React from 'react';
import { View, StyleSheet, Modal, Button } from 'react-native';
import Constants from 'expo-constants';
import Animated from 'react-native-reanimated';
import {FadeIn} from 'react-native-reanimated';

export default function App() {
  const [modalVisible, setModalVisible] = React.useState(false)
  const [viewVisible, setViewVisible] = React.useState(false)
  const onCloseClick = () => {
    setModalVisible(!modalVisible)
    setViewVisible(!viewVisible)
  }

  return (
    <View style={styles.container}>
      {
        !!viewVisible && (
          <Animated.View 
            entering={FadeIn.duration(300)} 
            exiting={FadeIn.duration(300)} 
            style={{ width: 100, height: 100, backgroundColor: 'blue' }
          }/>
        )
      }
      <Modal animationType="slide" transparent visible={modalVisible} onRequestClose={onCloseClick}>
        <View style={{width: 100, height: 100, backgroundColor: 'red'}} />
        <Button onPress={onCloseClick} title="CLOSE" />
      </Modal>
      <Button onPress={onCloseClick} title="OPEN" />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    paddingTop: Constants.statusBarHeight,
    backgroundColor: '#ecf0f1',
    padding: 8,
  },
});