Add namespace required by newer Gradle version

[DavidBertet]

Summary

While testing to update the example app to a newer ReactNative/Gradle version (for #589), I noticed Gradle requires a namespace to be defined.

Updating the example app will be done in another PR as it requires more testing, but adding the namespace could unblock some that are using it with a newer Gradle versions

There is a warning saying we can remove it from AndroidManifest.xml, but I wonder if that could break for users having a very old stack

package="com.rncamerakit" found in source AndroidManifest.xml: react-native-camera-kit/android/src/main/AndroidManifest.xml.
Setting the namespace via a source AndroidManifest.xml's package attribute is deprecated.
Please instead set the namespace (or testNamespace) in the module's build.gradle file, as described here: https://developer.android.com/studio/build/configure-app-module#set-namespace

How did you test this change?

The example app runs, shouldn't have any impact on the logic

[DavidBertet]

Turns out #617 does the same, with an additional check on Gradle version. Let's merge that one instead