[expo-router] Fix modal with detents fixToContents on Android (expo#38440)

# Why

When modal was used with presentationStyle `formsheet` and detents
`fitToContents`, the on Android it took all available space, instead of
just contents.

## Before


https://github.com/user-attachments/assets/cdb5ce21-4470-4603-960b-6f5c54e72420

## After


https://github.com/user-attachments/assets/67225dac-14fd-4a33-b5db-92084f94c8fb

# How

1. When presentationStyle is `formsheet` and detents are
`fitToContents`, the flex property is set to `undefined`

# Test Plan

1. Manual testing on Android and iOS

# Checklist

<!--
Please check the appropriate items below if they apply to your diff.
-->

- [x] I added a `changelog.md` entry and rebuilt the package sources
according to [this short
guide](https://github.com/expo/expo/blob/main/CONTRIBUTING.md#-before-submitting)
- [ ] This diff will work correctly for `npx expo prebuild` & EAS Build
(eg: updated a module plugin).
- [ ] Conforms with the [Documentation Writing Style
Guide](https://github.com/expo/expo/blob/main/guides/Expo%20Documentation%20Writing%20Style%20Guide.md)

Author: Ubax

packages/expo-router/CHANGELOG.md

@@ -34,6 +34,7 @@
 - Add static rewrites support to export and server-side handling ([#37930](https://github.com/expo/expo/pull/37930) by [@hassankhan](https://github.com/hassankhan))
 - Fix old arch build with LinkPreview code ([#38305](https://github.com/expo/expo/pull/38305) by [@Ubax](https://github.com/Ubax))
 - [iOS] Add compiler flags for new arch to expo router ([#38397](https://github.com/expo/expo/pull/38397) by [@Ubax](https://github.com/Ubax))
+- Fix modal with detents fixToContents on Android ([#38440](https://github.com/expo/expo/pull/38440) by [@Ubax](https://github.com/Ubax))
 
 ### 💡 Others
 

packages/expo-router/build/modal/ModalsRenderer.js

@@ -42,7 +42,7 @@ export const ModalsRenderer = ({
           }}
           contentStyle={[
             {
-              flex: 1,
+              flex: config.presentationStyle !== 'formSheet' ? 1 : undefined,
               backgroundColor: config.transparent ? 'transparent' : 'white',
             },
             config.viewProps?.style,