Os: add GetControlSocket()

Add the GetControlSocket() method to Os, and
add unit tests for the same.

A "control socket" is a service-specific socket
created by the init daemon, to enable other
processes to talk to a given service.

Bug: 32446680
Test: ./runtests.sh (on angler)
Change-Id: I25ea54f8ab85fafd492ec2953e4e369d64ecd02d
8 files changed