init: make system properties more secure.

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.

Modify system property handling so that we get the properties
from a root owned properties file, rather than using an
environment variable.

Related to bug: 8029617

Change-Id: I18610128e11c4037ed6f4dec6eba20f69fa647eb