How about this:
1) Magnet, that can be detected by phone (should be possible, VR cardboard uses such) when it is slided out.
2) You need something that wakes up the phone from sleep, e.g. front proximity sensor
3) You need something that triggers camera-app active

If magnet is there, and proximity is removed, launch camera app. Do nothing if magnet isn't there.

Solve the 1) and rest is easy