Add a system property test before scanning touch video devices.

EventHub opens and requests buffers for all touch video devices that
it finds on the system. As V4L does not allow multiple clients, this
prevents anyone else from using them.

This property permits touch video devices to be accessible to other
clients for testing/development.

Fixes: 122690279
Test: toggle property; restart services; examine lsof /dev/v4l-touch*
Change-Id: I46a3d9980c8fb1a241e4791709f2f4c07f8f3987