Add system captions manager service

This service connects to a remote system captions manager service. This
service is responsible for enabling system captions when the user
requests them. As the system binds to it, this service will be
persistent.

Bug: 128925852
Test: Manual. I created an implementation of the service.
Change-Id: Iafde1bb68f4754d8167624f47c6833d43c0ec336