property_service: make /dev/__properties__ readable

Currently, system properties are passed via the environment
variable ANDROID_PROPERTY_WORKSPACE and a file descriptor passed
from parent to child. This is insecure for setuid executables,
as the environment variable can be changed by the caller.

Make the /dev/__properties__ file accessible, so an app can
get properties directly from the file, rather than relying on
environment variables.

Preserve the environment variable for compatibility with pre-existing
apps.

Bug: 8045561
Change-Id: If81e2b705eca43ddd8b491871aceff59c0e00387