Support fine grain read access control for properties

Properties are now broken up from a single /dev/__properties__ file into
multiple files, one per property label.  This commit provides the
mechanism to control read access to each of these files and therefore
sets of properties.

This allows full access for all domains to each of these new property
files to match the current permissions of /dev/__properties__.  Future
commits will restrict the access.

Bug: 21852512

Change-Id: Ie9e43968acc7ac3b88e354a0bdfac75b8a710094