[android] [native-modules-android.md] consistent code for adding package to MainApplication.kt

[ChronSyn]

Updated the documentation regarding adding packages to MainApplication.kt, so that they are consistent with https://reactnative.dev/docs/native-components-android

Further reasoning The method listed in the pre-commit version did not correctly register the module in my testing, but the version in the proposed change does appear to work as expected

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[ChronSyn]

Thanks for sending this over @ChronSyn

Updated the documentation regarding adding packages to MainApplication.kt, so that they are consistent with reactnative.dev/docs/native-components-android

We should instead update the other page to follow the same pattern as in the template:

override fun getPackages(): List<ReactPackage> =
    PackageList(this).packages.apply {
        // Packages that cannot be autolinked yet can be added manually here, for example:
        // packages.add(new MyReactNativePackage());
        add(MyAppPackage())
    }

This looks to match the pre-commit version. That didn't appear to work correctly for me. It seemed that the return type was the cause of the problems, but I wasn't sure why. However, after retesting, I've confirmed that wasn't the issue so I've reintroduced it.

I've made an amendment to your suggestion - new MyReactNativePackage() is not valid for Kotlin and will cause the native module to be null on the JS side.

Further context of why this PR was opened is available at https://github.com/facebook/react-native/pull/41856 . That mentions the same issue, but it doesn't look like a PR was raised for the website docs to be corrected.

I've updated both files to be consistent.

[cortinico]

@ChronSyn can you also backport your change to the versioned docs?