I am not sure if you would like to build your firmware from source or just replace Launcher in runtime. If you are looking to replace the Launcher in source, you can use LOCAL_PACKAGES_OVERRIDES := Launcher2 Launcher3
If you are looking to replace the Launcher in runtime, you can remove the existing Launcher APK and replace it with another. The Android OS does not look for a specific package name to be default launcher. Instead, it will use anything with the Launcher permissions in the application manifest. If there are multiple applications that fit this criteria, the OS will prompt you to decide which to use.