[native] Use currently selected theme for alerts.

Summary:
Solution for [ENG-3935](https://linear.app/comm/issue/ENG-3935/consider-using-dark-mode-in-alerts-on-native).

This diff introduces a wrapper for React Native alerts, which synchronizes currently selected theme in the app with alert style. Right now `userInterfaceStyle` is only supported on iOS.

Light theme alert:
{F646510}
Dark theme alert:
{F646511}

Test Plan:
1. Select a location where you want to implement an alert (for example, you can add `useEffect` in `root.react.js`).
2. Verify whether the alert theme is consistent with the app theme.
3. Navigate to `profile > appearance`, alter the theme, and check if the same transformation applies to the other theme.

Reviewers: bartek, tomek, michal, inka

Reviewed By: tomek, inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8586

native/utils/alert.js

@@ -0,0 +1,48 @@
+// @flow
+
+import { Alert } from 'react-native';
+
+import { store } from '../redux/redux-setup.js';
+import { type GlobalTheme, defaultGlobalThemeInfo } from '../types/themes.js';
+
+type AlertWrapper = {
+  +alert: typeof Alert.alert,
+  +prompt: typeof Alert.prompt,
+};
+
+const getCurrentTheme = (): GlobalTheme =>
+  store.getState().globalThemeInfo.activeTheme ||
+  defaultGlobalThemeInfo.activeTheme;
+
+const alertWrapper: AlertWrapper = {
+  alert(title, message, buttons, options) {
+    Alert.alert(title, message, buttons, {
+      userInterfaceStyle: getCurrentTheme(),
+      ...options,
+    });
+  },
+  prompt(
+    title,
+    message,
+    callbackOrButtons,
+    type,
+    defaultValue,
+    keyboardType,
+    options,
+  ) {
+    Alert.prompt(
+      title,
+      message,
+      callbackOrButtons,
+      type,
+      defaultValue,
+      keyboardType,
+      {
+        userInterfaceStyle: getCurrentTheme(),
+        ...options,
+      },
+    );
+  },
+};
+
+export default alertWrapper;