To catch issues like: crbug.com/642553


Automated test procedure:

1. c920/c930 connected to the servo USB port so it can be hotplugged
2. Enable USB port such that the device can see the attached camera
3. Enroll device to CFM and join session
4. Simulate unplug and replug using the servo while in session


Verification:
1. Check to crashes at various stages in the test similar to https://chromium-review.googlesource.com/c/481048/
2. Check for camera in cras_test_client --dump_server_info output
3. Check for preferred camera by calling the hotrod API get_preferred_camera
4. Check for any camera connect / disconnect logs in kernel logs (similar to crbug.com/642553#c16)