Developer Preview builds of Android always add in extra features, sometimes experimental, sometimes refinements of existing functionality. It seems in the Android N Developer Preview 2 build, Google has decided to add in support for native VR.
The first hint at native VR support in Android N is hidden in the special access section in the settings app where a new entry called VR Helper Services is listed. It seems that this setting can be used by apps that can self-identify as VR Helpers and request access to the API to provide headset popups in VR mode, which seems to be referenced in the Android N source code as well as something called VR Listener.
Native support in Android N would certainly make sense and pave the way for a dedicated Google VR headset, which has been rumored as being in the pipeline.
Whilst still tethered by definition, mobile VR headsets could prove more popular in that they are more portable and the hardware they are tethered to is much more portable. Adding support for native VR in Android N would make official support of Android for mobile headsets manufactures extremely well placed.