Fix expensive view lifespans that are continuing in the background

[skgrush]

At the very least, the ScannerModel#arView is living in the background even when not in use. This indicates two potential problems: (1) we may be using the camera in the background unnecessarily, which is not great for resources and possibly privacy; (2) what other views might we be leaving around when unused, is my understanding of views fundamentally flawed?

• [x] answer: is the scanner ARView sticking around and using resources?
• [x] address: if possible, preventing camera use in the background / when not needed.
• [ ] answer: how are views / controllers supposed to be use used and destroyed?

[skgrush]

Answer to the first point, not only is the ARView sticking around and using the camera, it's also using significant resources even while the UI seems completely passive! So this absolutely needs to be addressed.

[skgrush]

In 1ef7b16 I prevented the ARView instance from persisting in the background, which clears up the confusion of using the camera in the background but still seems to keep a good chunk of memory allocated after it should have deallocated.