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...
For the people having problems with the "Ok Google" hotword:
1. Download the APK: http://wamod.ml/xda/opa/HotwordEnrollment.apk
2. Move it to /system/priv-app/HotwordEnrollment/
Report if works or not please. If it works I'll add it to FAQ.
I also tried to use a modified APK with no success, logcat throws SecurityException :silly:
What about disabling the signature check? I'll take a look on AOSP/GApps later.
EDIT: It's an stupid idea anyways. We need to keep the stock /system untouched, and seems impossible.
For me still works across reboots with only the OPA (ro.opa.eligible_device) line.
Oh, I don't know why, but Google uses the term "OPA" for the personal assistant on the source code of the Google app. Just a fact.
Adding only the line ro.opa.eligible_device=true Google Assistant works for a few minutes only. To keep it working you need to replace the device model.
EDIT: Seems like now it's working. (Maybe because I booted for the first time with the Pixel build.prop?) The download link is now updated.
Android 7.0 or higher
SuperSU or kernel with dm-verity disabled
Latest Google app (126.96.36.199 or newer)
Method 1: Manual editing
Just add or replace these lines on your build.prop:
Method 2: Flashable ZIP by...
Google Assistant for the Nexus 6P
New guide available here: http://forum.xda-developers.com/android/software/guide-how-to-enable-google-assistant-t3477879
Old patch for NBD90X
WARNING! Be sure you're using STOCK NBD90X System Image before continuing.
Method 1 UPDATED (Working, no bugs)