How to build a DiY Google Home from an old Android Phone?

When the phone is locked & screen off, it will never hear my voice. How do I get OK google to work when the phone is off? The option seems removed for non Google Pixel/Nexus phones. I want to convert my old android phone into a Google Home (so I can say "Ok Google, turn off the lights" in my bedroom). I have an old Samsung Note 2. I used Odin to put TWRP on it so I can run LineageOS w/ Gapps, and now I have Google Assistant running on it. However, when the phone is off, it will never hear my voice. I can turn on and off the smart lightbulb in my room with my voice now. But there is no "always listening" "wake word" for "Ok Google Everywhere" anymore. Pretty sure Google removed this feature so you have to buy a Google Home device. I want to be able to use Ok Google anywhere, even when charging my phone.