Refine rules for accessing /dev/__properties__

Don't allow processes to list out the contents of the directory
/dev/__properties__. This is an implementation specific detail that
shouldn't be visible to processes.

Test: Device boots and no problems reading individual properties.
Test: ls -la /dev/__properties__ fails
Change-Id: I4df6a829b0d22e30fb2c38030c690fc4a356f6a3