init: Document which class Service members are not modified

It is nontrivial to derive from the implementation of class Service
which members are not modified. Hence this CL that documents this by
declaring these members 'const'.

Change-Id: I27b907a1c7044376d5c5393a29050c66cbdab7bf
Signed-off-by: Bart Van Assche <bvanassche@google.com>