As you can see in the screen capture above, my test device (an unrooted, bootloader locked Google Nexus 6 device on Android 7.0 Nougat) has the standard set of nav bar keys until music playback is initiated in Google Play Music. (xda-developers.com)
While it's definitely possible to change nav bar icons using the ADB method or Custom Navigation Bar app, the end result is that you lose the ability to do long presses on the buttons. (xda-developers.com)
It was only recently, though, that we discovered that this hidden nav bar tuner in Android Nougat can actually be accessed without needing root access, a custom ROM, or System UI mods. (xda-developers.com)