Make `WritableMapBuffer` internal (#49914)

Summary:
This class can be internalized as part of the initiative to reduce the public API surface. I've checked there are [no relevant OSS usages](https://github.com/search?type=code&q=NOT+is%3Afork+NOT+org%3Afacebook+NOT+repo%3Areact-native-tvos%2Freact-native-tvos+NOT+repo%3Anuagoz%2Freact-native+NOT+repo%3A2lambda123%2Freact-native+NOT+repo%3Abeanchips%2Ffacebookreactnative+NOT+repo%3AfabOnReact%2Freact-native-notes+NOT+user%3Ahuntie+NOT+user%3Acortinico+NOT+repo%3AMaxdev18%2Fpowersync_app+NOT+repo%3Acarter-0%2Finstagram-decompiled+NOT+repo%3Am0mosenpai%2Finstadamn+NOT+repo%3AA-Star100%2FA-Star100-AUG2-2024+NOT+repo%3Alclnrd%2Fdetox-scrollview-reproductible+NOT+repo%3ADionisisChytiris%2FWorldWiseTrivia_Main+NOT+repo%3Apast3l%2Fhi2+NOT+repo%3AoneDotpy%2FCaribouQuest+NOT+repo%3Abejayoharen%2Fdailytodo+NOT+repo%3Amolangning%2Freversing-discord+NOT+repo%3AScottPrzy%2Freact-native+NOT+repo%3Agabrieldonadel%2Freact-native-visionos+NOT+repo%3AGabriel2308%2FTestes-Soft+NOT+repo%3Adawnzs03%2FflakyBuild+NOT+repo%3Acga2351%2Fcode+NOT+repo%3Astreeg%2Ftcc+NOT+repo%3Asoftware-mansion-labs%2Freact-native-swiftui+NOT+repo%3Apkcsecurity%2Fdecompiled-lightbulb+com.facebook.react.common.mapbuffer.WritableMapBuffer).

## Changelog:

[INTERNAL] - Make com.facebook.react.common.mapbuffer.WritableMapBuffer internal

Pull Request resolved: https://github.com/facebook/react-native/pull/49914

Test Plan:
```bash
yarn test-android
yarn android
```

Reviewed By: javache

Differential Revision: D70884414

Pulled By: cortinico

fbshipit-source-id: de1a298f7c7d5da8edf029e8d4dd31a9b334b853