Implement dual-camera mobile client

[floe]

for "see-through AR mode"

[floe]

cf. https://webrtc.github.io/test-pages/src/multiple-video-devices/

[floe]

Note: many mobile devices can only use one camera (front or back) at the same time

[floe]

cf. https://developer.android.com/reference/androidx/camera/core/ConcurrentCamera

[floe]

Unlikely to work in the web client, might actually require a dedicated app.