AR: define initial target API based on investigations of ARCore and ARKit

AR is used differently depending on which device type it's used with (HMD vs phone/tablet). On android based devices like the Quest, there's also a split between different frameworks (OpenXR/ARCore). In the future, with the arrival of AndroidXR, this landscape might change and we'll see more utalization of ARCore on the HMDs as well.

On Apple device the same split is also found, but on the AVP ARKit is already used (the ARCore equivalent on Apple devices).