Fix crash when virtual camera is unregistered during active session.

Store weak_ptr to VirtualCameraDevice instance in VirtualCameraSession
to detect when the camera was removed as well as to make sure
it's not destroyed when VirtualCameraSession accesses
VirtualCameraDevice.

Bug: 301023410
Test: atest VirtualCameraTest (not closing the session)
Test: atest virtual_camera_tests

Change-Id: Ie87c91f6872b9241dddf64f1a4f1991eb54d3cda