A new commit in the Android Open Source Project (AOSP) repository suggests that facial authentication is coming to Android as a system level API. It could mean that from system point of view, Android P could well allow for facial authentication on devices, assuming of course that the hardware to do so is built into the device.
Currently, for manufactures to support facial authentication (like OnePlus for example), they have to build their own app and leverage existing APIs, mostly around the fingerprint authentication. That works, obviously, but it isn’t as optimized as it could be and it is dependent upon OEMs to build the supporting software & hardware to make it happen. This commit would more-or-less standardize things for Android.