feat: Make react-native-config work with white labels

[Ldoppea]

During build phase, react-native-config generates a .BuildConfig class from the project .env file

Then on runtime, it will try to reconstruct this classe name by querying its package name from app's resources using the following code

int resId = context.getResources().getIdentifier("build_config_package", "string", context.getPackageName());
className = context.getString(resId);

This would work only if the app's packageId and namespace are the same

But since we implemented the new white label mechanism, those values can now be different. This is the case for the MaBulle version that has io.cozy.flagship.mobile.mabulle as package name but io.cozy.flagship.mobile as namespace

In the previous code, the resource would be searched in getPackageName() which would return the packageId but the resource would be stored in the namespace, so build_config_package would not be found

The result is that the app won't find any config value (i.e. Safetynet API key and Sentry auth token)

To fix that we add a new instruction in the build.gradle file to set build_config_package in the packageId too

As our white label mechanism always set io.cozy.flagship.mobile as namespace, so we use this value for build_config_package

Related commit: d08fd4b