One of the new features that Google brought to the new Google Pixel 2 and Pixel 2 XL is the Always On Display. That feature joins a couple others like the Now Playing function for identifying music that is playing and some of the tweaks available in the camera app’s Portrait Mode. It is not surprise that these features are limited to only working on Google’s latest devices as a way to encourage buyers to invest in the Pixel 2 or Pixel 2 XL. Some signs point to the restriction eventually dropping, but until then some developers have figured out a way to enable the Always On Display on older Google smartphones including the Nexus 6P, the Pixel and the Pixel XL without root access being required.
The code to make the Always On Display accessible on older devices appears in the Android 8.1 Oreo Developer Preview 1. For the current Android 8.0 Oreo version of the operating system, the code that checks for whether a device is allowed to run the Always On Display was hardcoded. Looking forward though, Google is moving to check for a value passed by a parameter. With that in mind, developers figured out that they can use the OverlayManagerService (OMS) to return the correct value even on older devices. This is combined with the Substratum “theme manager” and a plugin called Andromeda to customize the display and resource overlays used by the device, including the lock screen and Always On Display.
Step by step directions and a video showing the install process and setup are available using the source link below. If you decide to pursue this option to enable an Always On Display on your older Google device, keep in mind the screens on those devices have not been optimized to use the low power doze state so power consumption is still not clear. The developers who figured this out have also noted there are some issues with waking the device from a sleep state with the Always On Display enabled that requires doubling up on power button or screen double-taps to power on. Occasionally a device may get stuck on a gray screen as well. The developers note that locking and relocking a device using the fingerprint scanner resolves these issues.
source: XDA Developers