init: Introduce symbolic names for certain constants

Make the code easier to read by introducing symbolic names for the
constants used by Service::Start() for communication between the parent
and child processes.

Bug: 213617178
Change-Id: I3e735e149682fa9df2ed57f75eb5a67d7c68bd92
Signed-off-by: Bart Van Assche <bvanassche@google.com>