SmartUrlInput: Use useFocusEffect.

Following the doc at
  https://reactnavigation.org/docs/use-focus-effect/.

Greg points out [1] that `useFocusEffect` seems to have the effect
of calling the callback even on the component's first mount (if the
screen is focused), whereas the old listener-based code would not.
That doesn't seem to have a practical effect, here; empirically, it
looks like the input gets focused anyway before this change. That's
probably due to the `autoFocus` prop we pass to the `TextInput`.

[1] zulip#4914 (comment).

src/common/SmartUrlInput.js

@@ -1,6 +1,7 @@
 /* @flow strict-local */
-import React, { useState, useRef, useCallback, useContext, useEffect } from 'react';
+import React, { useState, useRef, useCallback, useContext } from 'react';
 import { TextInput, TouchableWithoutFeedback, View } from 'react-native';
+import { useFocusEffect } from '@react-navigation/native';
 import type { ViewStyleProp } from 'react-native/Libraries/StyleSheet/StyleSheet';
 
 import type { AppNavigationProp } from '../nav/AppNavigator';
@@ -62,7 +63,6 @@ export default function SmartUrlInput(props: Props) {
     onChangeText,
     onSubmitEditing,
     enablesReturnKeyAutomatically,
-    navigation,
   } = props;
 
   // We should replace the fixme with
@@ -71,8 +71,6 @@ export default function SmartUrlInput(props: Props) {
   // this is probably down to bugs in Flow's special support for React.
   const textInputRef = useRef<$FlowFixMe>();
 
-  const unsubscribeFocusListener = useRef<(() => void) | void>(undefined);
-
   /**
    * The actual input string, exactly as entered by the user,
    * without modifications by autocomplete.
@@ -81,20 +79,14 @@ export default function SmartUrlInput(props: Props) {
 
   const themeContext = useContext(ThemeContext);
 
-  const { addListener } = navigation;
-  useEffect(() => {
-    unsubscribeFocusListener.current = addListener('focus', () => {
+  useFocusEffect(
+    useCallback(() => {
       if (textInputRef.current) {
         // `.current` is not type-checked; see definition.
         textInputRef.current.focus();
       }
-    });
-    return () => {
-      if (unsubscribeFocusListener.current) {
-        unsubscribeFocusListener.current();
-      }
-    };
-  }, [addListener]);
+    }, []),
+  );
 
   const handleChange = useCallback(
     (_value: string) => {