Two ways to do it:
1. Download a Terminal and run:
# mount -o rw,remount /system
Then try modifying the build.prop again.
2. Boot on TWRP, mount system, open Terminal, run "vi /system/build.prop", edit the file, save, reboot.
Looking for libraries and decompiling the framework and apps of the dump like crazy, looking for the code of Google Assistant. When I saw on the SystemUI code the call of a method to return the Intent of an Assistant installed on the device, I supposed that it wasn't deeply implemented on the...