[Fabric] Implement onFocus and onBlur

[acoates-ms]

Description

Implements onFocus and onBlur for fabric. This adds the events to the ViewProps, ViewEventEmitter structures.

Minor fix to add logic to clear the focused component when a component is removed from the tree. (fixes a crash on reload)
Minor fix to fix an issue where updates to borderColor do not by themselves update the border.

Testing

This UI now shows the focus as it moves around when using tab/shift+tab.

const FocusableViewThingy = () => {
  const [isFocused, setIsFocused] = React.useState<boolean>(false);

  return (
    <View
      style={{
        backgroundColor: isFocused ? 'red' : 'green',
        width: 50,
        height: 50,
      }}
      focusable={true}
      onFocus={() => setIsFocused(true)}
      onBlur={() => setIsFocused(false)}
    />
  );
};

export default class Bootstrap extends React.Component {
  render() {
    return (
      <View accessible={true}>
        <FocusableViewThingy />
        <FocusableViewThingy />
        <FocusableViewThingy />
        <FocusableViewThingy />
      </View>
    );
  }
}

AppRegistry.registerComponent('Bootstrap', () => Bootstrap);

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