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

(cherry picked from commit 7ece0a86)

Change-Id: I762da21ef4075f288745efed0ec7d16c2b71303c